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A 



HIGH-SCHOOL ASTRONOMY: 



IN WHICH THE 



DESCRIPTIVE, PHYSICAL, AND PRACTICAL 



ARE COMBINED, 



"WITH SPECIAL REFERENCE TO THE WANTS OF 



ACADEMIES AND SEMINARIES OF LEARNING. 



BY HIRAM MATTISON, A. M„ 

late professor of natural philosophy and astronomy in the falley 

seminary ; author of the prlmary astronomy j astronomical 

maps; editor of burritt's geography of the 

heavens, etc., etc. 







NEW YORK: ^ 

F. J. HUNTINGTON, 
AND MASON & LAW, 23 PARK ROW, 

OPPOSITE THE ASTOE-HOUSE. 



Entered according to Aot of Congress, in the year 1853, 

BY HIEAM MATTISON, 

In the Clerk's Office of the District Court of the United States for the Southern 

District of New York.. 






& 



^ 



>-' V-^ 



PREFACE. 



The design of this work is to furnish a suitable text-book of 
Astronomy for academies and seminaries of learning. Though sub- 
stantially a revised edition of the " Elementary Astronomy" so exten- 
sive and important have been the additions and improvements, as to 
justify the adoption of a new title, and to warrant the hope that it 
will not only be found eminently suited to its purpose, but that it 
may now go on for years without further revision or alteration. 

For juvenile learners, the " Primary Astronomy" may be preferred ; 
and for advanced classes, who wish to study the constellations in 
connection with Mythology, the " Geography of the Heavens" should 
be chosen in preference to all others ; but for all ordinary students, 
this intermediate work will be found sufficiently elementary on the 
one hand, and sufficiently extended on the other. 

The work is now divided in three parts. After an Introduction, 
which consists of Preliminary Observations and Definitions, and occu- 
pies twenty pages, Part First is devoted to the Solar System — the sun, 
planets, comets, eclipses, tides, &c. ; Part Second relates to the Sidereal 
Heavens — the fixed stars, constellations, clusters, and nebulas; and 
Part Third to Practical Astronomy — the structure and use of instru- 
ments, refraction, parallax, &c. This department, so seldom intro- 
duced into text-books for schools, will be found especially interesting 
and valuable. 

Besides embracing all the late discoveries in astronomy, under a 
strictly philosophical classification, the work is now thoroughly illus- 
trated, by the introduction of diagrams into the pages, in connection 
with the text ; and the adaptation throughout to the use of the black- 
board, during recitation, cannot fail to be appreciated by every prac- 
tical teacher. 

The variety of type affords an agreeable relief to the eye of the 
student, and at the same time distinguishes the main text (which 
ought, in all cases, to be thoroughly understood before it is passed) from 
the less important matter, the more careful study of which may be 
left for a review. The suggestive topical questions at the bottom of 
the page complete the design. 

On the whole, the work is believed to be a decided improvement 
upon the works heretofore in use in this department of study; and 
as such it is offered to the professional teachers of the country. 

New York, Jan. 1, 1853. 



ASTRONOMICAL WORKS 

In the Author's Library, and more or less consulted in the compilation 

of the following pages : 

A Cycle of Celestial Objects, for the use of Naval, Military, and Private Astronomers, 
&c. By Capt. Wm. Henry Smyth, &c. 2 vols. Svo. London, 1S44. 

An Introduction to Astronomy, in a Series of Letters from a Preceptor to his Pupil, 
&c. By John Bonnycastle, Professor of Mathematics, &c. 1 vol. Svo. London, 
1822. 

An Introduction to the True Astronomy ; or, Astronomical Lectures read in the 
Astronomical School of the University of Oxford. By John Keill, M. D., F. E. S., 
&c. 1 vol. Svo. Dublin, 1793. 

Astronomy Explained, upon Sir Isaac Newton's principles, &c, &c. By James Fer- 
guson, F. K. S. 1 vol. 4to. London, 1764. 

TJie Elements of Physical and Geometrical Astronomy. By David Gregory, M. D., 
late Sullivan Professor of Astronomy at Oxford, &c. 2 vols. Svo. • London, 1726. 

Astronomy, in Five Books. By Eoger Long, D. D., F.E. S., &c, University of Cam- 
bridge. 2 vols. 4to. Cambridge (Eng.), 1742. 

Astronomia Carolina, &c, by Thomas Street ; and A Series of Observations on the 
Planets, chiefly the Moon, &c, by Dr. Edmund IIalley. 1 vol. 4to. London, 1716. 

Astronomical Lectures, read in the Public School at Cambridge (Eng). By William 
Whiston, M. A., Professor of Mathematics, &c. 1 vol. Svo. "London, 172S. 

TJie Wonders of the Heavens ; a popular view of Astronomy, &c. By Duncan Brad- 
ford. 1 vol. royal 4to. New lork, 1843. 

Popular Lectures on Science and Art, &c. By Dionysius Lardner, F. E. S., &c, 
&c. 2 vols. Svo. New York, 1846. 

Outlines of Astronomy. By Sir John F. W. Herschel, Bart, K. II., &c. 1 vol. Svo. 
Philadelphia, 1S49. 

Phenomena and Order of the Solar System, and Views of the Architecture of tJie 
Heavens. By J. P. Niciiol, F. E. S. E., &c. 2 vols. 12mo. New York, 1S42. 

The Practical Astronomer, &c. By Thomas Dick, LL.D. 1 vol. 12mo. New York, 
1S46. Also, " Celestial Scenery," and " The Sidereal Heavens," by the same author. 

TJie Planetary and Stellar Worlds. By Prof. O. M. Mitchel. 1 vol. 12mo. New 
York, 1849. 

An Elementary Treatise on Astronomy, &c. By William A. Norton, A. M. 1 vol. 
8vo. New York, 1S45. 

An Introduction to Astronomy, &c. By Denison Olmsted, A. M. 1 vol. Svo. New 
York, 1844. Also, Letters on Astronomy, and Life and Writings of Ebenezer Por- 
ter Mason, by the same author. 2 vols. 12mo. 

The Solar System; or, the Sim, Moon, and Stars. By J. E. Hind, Director of Mr. 
Bishop's Observatory, Eegent's Park, London. 1 vol. 12mo. London, 1S52. 

A Pictorial Display of the Astronomical Phenomena of the Universe, &c. By C. F. 
Blount. 4to. New York, 1S44. 

The Recent Progress of Astronomy, &c. By Elias Loomis, Professor of Mathematics, 
&c. 1 vol. 12mo. New York, 1S50. 

Annual of Scientific Discovery, &c. By David A. Wells, A. M. 1 vol. 12mo. Bos- 
ton, 1S52. 

TJie Sidereal Messenger ; a Monthly Journal, devoted to Astronomical Science. By 
O. M. Metchel, A. M. (Now discontinued.) 

Also, Astronomical Lectures by Arago, Lardner, Mitciiei,, and Nichol; and Ele- 
mentary Treatises by Burritt, Kendal, Bartlet, MoIntire, Abbott, Qstrandek, 
Blake, Hasler, Smith, Clark, Yose, Tyler, Comstock, Haskins, Eyan, Wilkins, 
Keatii, &c, &c. 



CONTENTS, 



INTRODUCTION. 

. PRELIMINARY OBSERVATIONS AND DEFINITIONS. 

PAGE 

Chap. I. — Origin and History of the Science. 

Ptolemaic Theory of the Structure of the Universe 12 

The Copernican System 13 

II. — Definitions. 

Solids, Surfaces, &c 16 

Spheres, Hemispheres, and Spheroids 17 

Lines and Angles - 19 

Of Triangles 20 

Circles and Ellipses 21 

The Terrestrial Sphere 23 

The Celestial Sphere 25 

First Grand Divisions of the Universe 28 



PART FIRST. 

THE SOLAR SYSTEM. 

Chap. I. — The Primary Planets. 

Classification of the Solar Bodies 29 

Names of the Primary Planets 31 

Explanation of Mythological Signs 32 

Distances of the Planets 36 

Light and Heat of the Planets 38 

Magnitude of the Planets 40 

Density 41 

Gravitation 42 

Periodic Revolutions of the Planets 44 



CONTENTS. 



PAGE 

Chap. I. — Hourly Motion of the Planets in their Orbits 45 

Centripetal and Centrifugal Forces 45 

Laws of Planetary Motion 46 

Aspects of the Planets 48 

Sidereal and Synodic Revolution 49 

The Ecliptic, Zodiac, Signs, <fcc 50 

Celestial Latitude and Longitude 53 

Mean and True Places of a Planet 54 

Direct and Retrograde Motions. I 55 

Morning and Evening Stars 5*7 

Deviation of the Orbits of the Planets from the Ecliptic 58 

Philosophy of Transits 60 

II. — Primary Planets Continued. 

Inclination of the Axis of the Planets, and its Effects. . 65 

Rotation of the Planets upon their axes 69 

Time 70 

Equation of Time 72 

Time, as affected by Longitude 76 

True Figure of the Planets 77 

Precession of the Equinoxes 80 

III. — Telescopic Views of the Planets. 

Mercury — Phases, Mountains, <fcc 83 

Venus — Phases, Mountains, Atmosphere 84 

Mars — " Continents and Seas," Color, Snow Banks 86 

The Asteroids — Color, Hazy Appearance 87 

Jupiter — Oblateness, Belts, Moons 88 

Saturn — Oblateness, Rings, Belts, Phases, Moons 89 

Uranus — a Telescopic World, Satellites 94 

Neptune — purely Telescopic, Satellite 95 

Herschel's Solar System in Miniature 95 

IV. — Seasons of the Different Planets, <fcc. 

Cause of the Seasons — Mercury, Earth 97 

Venus, Mars, Jupiter 98 

Saturn and Uranus , 99 

Discovery of the different Planets 100 

V. — Secondary Planets — The Moon. 

Character and Number of the Secondaries 102 

The Moon's Distance, Shape, Position of Orbit, <fec 103 



CONTENTS. 



PAGE 

Chap. V. — Magnitude, Density, Revolution Eastward 105 

Form of Lunar Orbit 107 

Cause of the Moon's Changes 109 

Natural appearance — Same side always toward us. . . . Ill 

Moon's Librations — in Latitude and Longitude 112 

Telescopic Appearance of the Moon— Lunar Mountains 113 

Finding the Longitude by the Moon's place 115 

VI. — Eclipses of the Sun and Moon. 

Philosophy of both 116 

Law of Shadows 117 

Why not two Eclipses every Lunar Month 118 

Why Solar pass eastward over the Sun, and Lunar west- 
ward over the Moon 119 

Ecliptic Limits — Umbra and Penumbra 120 

Why all Central Eclipses not total 122 

VII. — Satellites of the Exterior Planets. 

Satellites of Jupiter — Distances, Periods, &c 124 

Eclipses of Jupiter's Moons — Immersions and Emersions 126 

Moons of Saturn — Why seldom eclipsed 127 

Satellite of Neptune 129 

VIII. — Nature and Cause of Tides. 

Description of Tides, Causes, (fee 130 

Spring and Neap Tides 134 

IX. — Of Comets. 

Name, Parts, Orbits, <fec i. 136 

Magnitudes, Velocity, Temperature, Periods 140 

Numbers, Physical Natures, <fec 143 

X.— The Sun. 

True Figure, Spots 146 

Physical Constitution, Temperature 151 

Zodiacal Light 153 

Sun's Proper Motion in Space 155 

XL — Miscellaneous Remarks upon the Solar System. 

Nebular Theory of its Origin 156 

Were the Asteroids originally one Planet ? 159 

Are the Planets inhabited by rational beings ? 161 



8 CONTENTS. 



PART SECOND. 

THE SIDEREAL HEAVENS. 

PAGE 

Chap. I. — The Fixed Stars. 

Classification of the Stars 166 

Number of the Stars 168 

Distances of the Stars 170 

II. — Description of the Constellations. 

Nature, Origin, Classification 172 

Visible in October, November, and December 174 

" January, February, and March 177 

" April, May, and June 180 

" July, August, and September 183 

III. — Double, Variable, and Temporary Stars, <fcc. 

Stars Optically and Physically Double 187 

Binary Systems 189 

Variable or Periodical Stars 194 

Temporary Stars — New and Lost 196 

IV. — Clusters of Stars and Nebulae. 

Pleiades, Hyades, &c 199 

Nebulae — Resolvable, Irresolvable, Aunular, &c 201 

Planetary, Stellar, &c 203 

Star Dust, Milky Way 206 



PART THIRD. 

PRACTICAL ASTRONOMY. 

Chap. I. — Properties of Light. 

Refraction of Light 211 

Atmospherical Refraction 214 

Refraction by Glass Lenses 216 

II. — Telescopes. 

Refracting Telescopes 221 

Reflecting Telescopes 231 

Transit Instrument 235 

Mural Circle 236 

Parallax 237 

Meteors and Meteoric Stones 239 



INTRODUCTION. 
PRELIMINARY OBSERVATIONS AND DEFINITIONS. 



4 ♦ » 



CHAPTER I. 

ORIGIN AND HISTORY OF THE SCIENCE. 

1. Science is knowledge systematically arranged, so 
as to be conveniently taught, easily learned, and readily 
applied. 

2. Astronomy is the science of the heavenly bodies — 
the Sun, Moon, Planets, Comets, and Fixed Stars. 

The term astronomy is from the Greek astron, a star, and nomos, a law ; and sig- 
nifies the laws or science of the stars. 

3. Astronomy is divided into Descriptive, Physical, 
and Practical. 

Descriptive Astronomy includes the mere facts of the 
science, irrespective of the causes of the phenomena ob- 
served, or of the means by which the facts were ascer- 
tained. 

Physical Astronomy explains the causes of the vari- 
ous phenomena observed, as of Day and Night, the 
Seasons, Eclipses, Tides, &c. 

Practical Astronomy relates to the means for acquiring 
astronomical knowledge by the use of instruments, and 
by mathematical calculations. 

These three departments have arisen, one after the other, in the order in which they 
are here stated. At first a few facts and phenomena were observed, but the causes 
were unknown. Next some of the causes were investigated one by one; and, finally, 
instruments were invented for measuring distances, altitudes, &c. ; data for calcula- 
tions were obtained; and thus arose the department of Practical Astronomy. 

1. Define the term Science. 

2. What is Astronomy ? (From what is the term derived ?) 

3. How is astronomy "divided? Descriptive? Physical? Practical? (State 
the order in which these departments have arisen.) 

i* 



10 ASTK0N0MY. 



4. Astronomy has long been regarded as the most 
sublime of the sciences, eminently calculated to illus- 
trate the wisdom, power, and goodness of God ; to ele- 
vate and expand the human mind, and to fill it with ex- 
alted views of the Creator — 

" The glorious Architect who built the skies." 

1. "The greatest men of all ages have pronounced this science to be the most sublime 
and surpassing of all that can be tested by human genius, and to be worthy of a life of 
study." — Smyth's Celestial Cycle. 

2. " Our very faculties are enlarged with the grandeur of the ideas it conveys, our 
minds exalted above the low-contracted prejudices of the vulgar, and our understand- 
ings clearly convinced, and affected with the conviction of the existence, wisdom, power, 
goodness, and superintendency of the Supreme Being!" — Ferguson. 

3. So remarkably does this science exhibit the glory and majesty of God, by its 
astounding revelations of His ivorks, that it almost necessarily tends to fill the mind 
with awe and reverence. It was in view of this tendency that the poet Young said, 

" An undevout astronomer is mad." 

4. To the moral influence of the contemplation of the heavens, we have frequent 
reference in the sacred Scriptures. "The heavens declare the glory of God" and the 
firmament showeth his handy-work." (Psalm xix. 1.) "When I consider thy heavens, 
the work of thy fingers; the moon and stars, which thou hast ordained; what is man. 
that thou art mindful of him? and the son of man, that thou visitest him?" (Psalm 
viii. 3, 4.) 

5. Astronomy is probably the most ancient of all the 
sciences. Some of the Chaldean observations date as 
far back as 2,250 years before Christ, or only 98 years 
after the Flood ! Laplace speaks confidently of Chinese 
observations 1,100 b. c. ; and Mr. Bailly, an English 
astronomer, fixes the time of a conjunction of Mars, 
Jupiter, Saturn, and Mercury, mentioned in Chinese 
records, at 2,449 years before Christ. 

1. The ancient Chinese astronomers and mathematicians were held to a fearful re- 
sponsibility for the- correctness of their calculations. In the reign of the Emperor Chou- 
kang, his two chief astronomers, Ho and H% were condemned to death for neglecting to 
announce the precise time of a solar eclipse, which took place B. 0. 2,169. 

2. The Holy Scriptures, some parts of which are very ancient, contain several allusions 
to the science of astronomy. In the first chapter of G.enesis we have an account of the 
creation of the Sun, Moon, and Stars. "And God said, Let there be lights in the firma- 
ment of the heaven, to divide the day from the night, and let them be for signs, and for 
seasons, and for days and years. And let them be for lights in the firmament of the 
heaven, to give light upon the earth : and it was so. And God made two great lights ; 
the greater light to rule the day, and the lesser light to rule the night: he made the 
stars" also." Verses 14-16. 

3. In the book of Job, written 1,500 years before Christ, we read of several constella- 
tions that bear the same names now that they did three thousand years ago. ""Which 
maketh Arcturus, Orion, and Pleiades, and the chambers of the south. " (ix. 9.) Again : 
" Canst thou bind the sweet influences of Pleiades, or loose the bands of Orion ? Canst 
thou bring forth Mazzaroth in his season? or canst thou guide Arcturus with his sons?" 
(Chap, xxxviii. 31, 32.) 

4. How astronomy regarded ? (Smyth ? Ferguson ? Young ? Scriptures ?) 

5. What of antiquity of astronomy'? Chaldean and Chinese observations? 
(Responsibility of Chinese astronomers ? Ancient Scriptural allusions ?) 






EARLY ASTRONOMERS. 11 



6. The first astronomers were shepherds and herdsmen, 
who were led to this study by observing the movements 
of the sun, moon, and stars, while watching their flocks 
from year to year in the open fields. 

ANCIENT ASTRONOMERS OBSERVING THE HEAVENS. 



7. Tholes, one of the seven wise men of Greece, was 
the first regular teacher of Astronomy, b. c. 600. The 
next was Anaximander, a disciple of Thales, who suc- 
ceeded him as head of the school at Miletus, b. c. 548. 
He asserted the true figure of the earth, and seems to 
have had some idea of its daily revolution. 

Anaximander is supposed to have been the first who constructed globes and maps. 
He taught that the moon shines by reflection, and in several other respects advanced 
beyond the knowledge imparted by his distinguished tutor. 

8. Pythagoras, another Greek philosopher, who 
founded the school of Croton, b. c. 500, greatly enlarged 
the science. He first gave form to the vague ideas that 
the sun was in the center of the planetary orbits, that 
the earth floated unsupported in space, and that the dis- 
tant stars were worlds, and probably inhabited. 

"It was Pythagoras, 11 says Smyth, "who taught, in fact, the system which now im- 
mortalizes the name of Copernicus." But he adds that his teachings were but "the con- 
jectures of a sagacious mind, not possessed of the evidence requisite to give stability to 
its opinions. " Pythagoras is said to have perished from hunger, in his old age. 

6. Who were the first astronomers ? How led to this study ? 

7. Who first regular teacher of this science ? How early ? Who next ? — 
and when ? What correct notions did he seem to entertain ? (For what else 
distinguished ?) 

S. Who next after Auaximander ? What advances did he make in this 
study ? ( What does Smyth say of his teachings ? What said of his death X) 



12 



ASTRONOMY 



9. Ptolemy, an Egyptian philosopher, taught astronomy 
in the second century of the Christian era. He adopted 
the theory that the earth was located in the center of the 
universe, that it was perfectly at rest, and that the sun, 
moon, and stars actually revolved around it, from east to 
west, as they appear to do, every twenty-four hours. This 
system is called, after its author, the Ptolemaic Theory. 



PTOLEMAIC THEORY OF THE STRUCTURE OF THE UNIVERSE. 




1. Ptolemy supposed the earth to he in the center of a system of crystalline arches, 
or hollow spheres, arranged one within the other, as represented in the cut. It is thought 
by some that he understood the spherical figure of the earth, and the cut is constructed 
upon this supposition. Ptolemy further supposed that the sun, moon, and stars were? 
fixed in these crystalline spheres, at diiferent distances from our globe ; that the Moon 
was in the first, Mercury in the second, Venus in the third, the Sun in the fourth, Mars, 

9. Who was Ptolemy ?— and when did he flourish ? Describe his theory, 
(How locate sun, moon, &e. 'I What absurdity did it involve, as it respects 



COPERNICAN SYSTEM. 13 



Jupiter, and Saturn in the next three, and the fixed stars in the eighth. The ancients 
had no knowledge of Uranus or Neptune. This ponderous machinery was supposed to 
revolve from east to west around the earth, carrying with it the sunj moon, and stars, 
every twenty-four hours; and the spheres being crystal, the distant stars were visible 
through them. 

2. If the sun was designed to enlighten and warm the different sides of our globe, 
the Ptolemaic method of effecting this object is most unreasonable. To carry the sun 
around the earth, to warm and enlighten its different sides, instead of having the earth 
turn first one side and then the other to the sun, by a revolution on its axis, would be 
like carrying a fire around a person who was cold, and wished to be warmed, instead of 
his turning himself to the fire as he pleased. 

3. The Ptolemaic theory would require a motion inconceivably rapid in all the 
heavenly bodies. As the sun is ninety-five millions of miles from the earth, the entire 
diameter of his sphere would be one hundred and ninety millions of miles,, and its cir- 
cumference about six hundred millions. Divide this distance by twenty -four — the num- 
ber of hours in a day — and it gives twenty-five million miles an hour, or sixty -nine 
thousand four hundred and forty-four miles per second, as the velocity of the sun ! This 
theory would require a still more rapid motion in the fixed stars. It would require the 
nearest of these to move at the rate of nearly fourteen thousand millions of miles per 
second, or seventy thousand times as swift as light, in order to accomplish their daily 
course. But with all these difficulties in its way, the Ptolemaic theory was generally 
believed till about the middle of the sixteenth century, or three hundred years ago. 

THE COPERNICAN SYSTEM. - 

10. About the year 1510, the ancient theory of Pythag- 
07"as was revived and improved by Copernicus, a Prus- 
sian astronomer, and has since been called, after him, the 
Copernican System. 

1. The investigations of Copernicus were conducted between the years 1507 and 15S0. 
In the latter year he finished his tables of the planets, and his great work, The Revolu- 
tion of the Celestial Orbs ; but he did not venture to publish his views till thirteen years 
after, or 1543, when he received a copy of it only a few hours before his death, and con- 
sequently never read it in print. It contains the old philosophy, interspersed with his 
own original and acute conceptions, and was received under very considerable opposi- 
tion. — Smyth, vol. 1, p. 38. 

2. Copernicus is generally regarded as the discoverer of the system which bears his 
name, but this is a popular error. There is abundant proof, notwithstanding the loss of 
his writings, that Pythagoras understood the leading features of what is now called the 
Copernican Theory. 

11. The first prominent feature of the Copernican sys- 
tem is, that the earth is a sphere or globe, inhabited on 
all sides. 

The evidence that the earth is a sphere or globe may be arranged and stated as fol- 
Ioavs : 

1. Admitting that the sur;, moon, and stars arc worlds, the fact that they are round, 
as we see them to be, affords ground for the presumption, at least, that the earth also is 
round. 

2. Water falling from the clouds is gathered into little globes or drops ; and melted 
lead poured from the summit of a high tower assumes the form of globes, which, when 
cooled, are called shot And the same law would cause a larger mass of fluid matter, if 
left undisturbed in space, to assume the same shape. But the Bible teaches that tho 

light and heat? "What in respect to the motions of the heavenly bodies? 
Was such a theory ever generally believed ? Till how recently ?) 

10. Who was Copernicus? For what distinguished? About what time? 
(What of his investigations? His work? Its publication? Character? 
What popular error noticed ?) 

11. State the first leading feature of the Copernican theory. (What 
proofs of its correctness ? The first ? Second ? Third ? Fourth ? Fifth ? 
Sixth 3 Seventh ?) 



14: ASTK0N0MY. 



whole earth was once in a fluid state — one vast drop — the substances now constituting 
the oceans and continents being indiscriminately mingled together. " And the earth 
was without form and void [i «,, chaotic, confused, unorganized], and darkness dwelt 
upon the face of the deep; and the spirit of God moved upon the face of the waters. 
* * * And God said, Let the waters under the heavens be gathered together unto 
one place, and let the dry land appear: and it was so. And'God called the dry land 
earth, and the gathering together of the waters called he seas." — Genesis i. 2, 9, 10. Up 
to this time there was no " earth," either as continents or islands, neither were there any 
"seas," but all the elements were mingled together; and a mass of fluid thus dropped 
into space, from the hand of the Creator, would be as certain to assume the form of a 
globe, as the melted lead from the shot-tower, or the water from the passing cloud. 

3. The apparent elevation and depression of the North Star, as we approach toward or 
recede from it, shows that the surface of the earth is convex, or that the earth is a globe. 

4. The fact that the tops of mountains are last seen as we recede from, or first as we 
approach, the sea-shore, proves that the surface of the water upon which we sail is con- 
vex ; so when a ship is approaching the shore, the topmasts are always seen first, and 
the hull or body last. And when seamen wish to survey the horizon at sea to a great 
distance, in search of whale or other shipping, they " go to the mast-head," as they call 
it, from which point they can often discover objects that are entirely invisible from the 
deck of the ships. 

5. If an aqueduct is to be constructed a mile long, so as to be filled with water to the 
brim at every point, it must be about eight inches higher in the middle than at the ends, 
so as to allow the surface of the water to conform to the convex figure of the globe. "We 
say higher, not that it needs to be higher as determined by a water level, for a water 
level is convex, but higher as determined by a straight line drawn from one end of the 
aqueduct to the other. This definite knowledge of the curvature of water, even for small 
distances, shows that the earth's surface is convex — or, in other words, that the earth is 
spherical. An inch of curve for every forty rods brings the evidence near home, and 
perfectly within our comprehension. 

6. When the moon falls into the shadow of the earth and is eclipsed, or, in other 
words, the earth gets into her sunlight, and throws its shadow upon her, the shadow is 
seen to be convex. We must either conclude, therefore, that the earth, which casts the 
shadow, is in the form of a dinner-plate, and is always kept sidewise, and the same side 
toward the sun (which Ave know is not the case); or that it is a globe, and casts a coni- 
cal shadow, whatever its position. 

7. The earth is known to be a globe, from the fact that ships arc constantly sailing 
around it. 

8. It is not certain whether Ptolemy admitted the earth to be a sphere or not. Some 
writers maintain that he rejected this doctrine, and others that he admitted it. In the 
" Primary Astronomy," page 8, the author has inserted a cut representing the Ptole- 
maic theory, with the earth Jlat; but in this work (page 12), where the same theory is 
represented, the earth is shown as a globe. In all other respects, the theory represented 
is the same in both works ; and this is only a minor point in the system. 

12. A second leading feature of the Coperfrican theory 
is, that the apparent revolution of the sun, moon, and 
stars westward every day, is caused by the revolution of 
the earth around its own axis, from west to east, every 
twenty-four hours. 

That the heavenly bodies appear to revolve westward, is no proof that they are actu- 
ally in motion. We often transfer our own motion, in imagination, to bodies that are at 
rest ; especially when carried swiftly forward without any apparent cause, as when one 
travels in a steamboat or railway car, and when for a time he forgets his own motion. 
" Copernicus tells us that he was first led to think that the apparent motions of the heav- 
enly bodies, in their diurnal revolution, were owing to the real motion of the earth in 
the opposite direction, from observing instances of the same kind among terrestrial ob- 
jects ; as when the shore seems to the mariner to recede as he rapidly sails from it, and 
as trees and other objects seem to glide by us, when, on riding swiftly past them, we lose 
the consciousness of our own motion." This remark would go to show that the revolu- 
tion of the earth on its own axis was an original discovery with Copernicus. 

12. State the second leading feature of the Copernican system. (Do not 
our own senses furnish proof that the heavenly bodies revolve westward 
daily? Why not ? W T hat remark from Copernicus ? . W T hat does it seem to 
imply ?) 



COPERNICAN SYSTEM. 



15 



13. A third feature of the Copernican theory is, that 
the sun is the grand center around which the earth and 
all the other planets revolve. 



THE COPERNICAN SYSTEM. 




1. The above cut is a representation of the Copernican, Theory of the Solar System. 
In the center is seen the sun. in a state of rest. Around him, at unequal distances. :>.ro 
the planets and fixed stars — the former revolving about him from west to east, or in the 
direction of the arrows. The white circles represent the orbits, or paths, in which the 
planets move around the sun. On the right is seen a comet plunging down into the sys- 
tem around the sun, and then departing. This is the Copernican Theory of the Solar 
System. 

" Ohow unlike the complex works of man, 
Heaven's easy, artless, unencumber'd plan !" 

2. The truth of the Copernican theory is established by the most conenssive and satis- 
factory evidence. Eclipses of the sun and moon are calculated upon this theory, and 
astronomers are able to predict thereby their commencement, duration, Asa, to a minute, 
even hundreds of years before they occur. We shall therefore assume the truth of this 
system without further proof, as we proceed hereafter to the study of the heavenly 
bodies. 

13. State the third prominent feature of the theory of Copernicus. (De- 
scribe the cut. What additional evidence of the truth of this theory, as ft 
whole ?) 



16 ASTRONOMY. 



CHAPTER II 



DEFINITIONS.* 



14. Solids, Surfaces, &c. 

A Solid, or Body, is a figure having length, breadth, 
and thickness. 

A Surface is the outside or exterior of a body, and has 
length and breadth only. 

Surfaces are of three kinds — Plane, Concave, and Con- 
vex. 

A surfaces may also be rough or smooth, hard or soft ; the above definition having 
reference only to the general figure of bodies. 

A Plane Surface is one that is perfectly flat or even, 
like the floor of a building, or the sides of a room. 

1. We may imagine what is called a plane, to extend off beyond the plane surface 
as far as we please ; or, in other words, to be indefinitely extended. When a plane or 
a line is extended in this way, it is said to be produced. 

2. An imaginary plane may exist where there is no body having -a plane surface ; or 
between two lines, like the plane of a circle. A sheet of tin, laid across a small wire 
hoop, would represent the plane of that circle, in whatever position it might be held, 
whether horizontally, perpendicularly, or otherwise; and the place which the tin would 
pass through, if extended to the starry heavens, is the plane of that circle. 

3. All objects which the tin would touch or cut, if extended outward to 
the heavens, or to infinity, are in the plane of the sheet, or the circle upon 
which it is laid. A point is in a plane produced, when the plane continued 
or extended would pass through that point. 

Parallel Planes are such as would never meet 
or cut each other, however far they might be ex- 
tended. 

The two sides of a board, or two sheets of tin placed equidistant from each 
other at every point, represent parallel planes. 



* To some who will use this work, many of the following diagrams and definitions will 
be superfluous, the substance of them being already sufficiently understood. With such 
students the judicious teacher will pass rapidly over the next ten pages, or omit them 
altogether. 

14. Define a solid,, or body — a surface. How many kinds of surfaces ? 
(Any other distinctions ?) What is a plane surface ? (May a plane extend 
beyond the plane surface ? May a plane exist where there is no body ? Il- 
lustrate. "W hat is a plane produced ?) What are parallel planes ? Perpen- 



DEFINITIONS. 



IT 



PERPENDICULAR PLANES. 




Perpendicular Planes are 
such as stand exactly upright 
upon each other, or cross each 
other at right angles. 

In the figure, one plane is placed horizontally, 
and the other perpendicular to it. They are 
therefore perpendicular to each other, however 
they may stand in relation to the observer. 

Inclined Planes are such as 
are inclined toward, and cut 
each other obliquely. 

The Angle of Inclination is 
the angle contained between the 
two surfaces of the planes near- 
est each other. 

The spaces A and B in the adjoining cut repre- 
sent the Angle of Inclination. 

The Area of a plane figure is the amount of surface 
contained therein. 



INCLINED PLANES. 




CONVEX AND CONCAVE SURFACES. 




A Convex Surface is one that 
is swollen out like the outside of 
a bowl. 

A Concave Surface is one that 
is hollowed out like the inside 
of a bowl. 

15. Spheres, Hemispheres, 
and Spheroids. 



A Sphere is a globe or lall^ every 
part of the surface of which is equidis- 
tant from a point within, called its 
center. 

This is the ordinary definition ; but in Astronomy, the term 
is applied to the apparent concave of the heavens, as if it were 
the actual concave surface of a hollow sphere. 



dicular ? Inclined ? What is meant by the angle of inclination ? The area 
of a plane surface ? Describe a convex surface — a concave, 

15. Describe a sphere — hemisphere — spheroid. (Derivation of spheroid ?) 



A SPHERE. 




18 



ASTK0N0MY. 



A HEMISPHER 




A Hemisphere is the half of a sphere or 
globe, or of the apparent concave of the heav- 
ens. 

In Geography we often read of the Eastern and Western, and North- 
ern and Southern hemispheres, but in Astronomy the term is only ap- 
plied to the Northern and Southern portions of the heavens. 



A Spheroid is a body resembling a sphere, but yet 
not perfectly round or spherical. 

The term spheroid is from the Greek sphaira, a sphere, and eidos, form, and signi- 
fies sphere-like. 

Spheroids are of two kinds — Oblate, ax oblate spheroid. 
and Oblong or Prolate. 

An Oblate Spheroid is a globe 
slightly flattened, as if pressed on oppo- 
site sides. 



This is a difficult figure to represent upon paper. Should 
the pupil fail to obtain a correct idea, the Teacher will be at 
no loss for an illustration. 

A Prolate or Oblong Spheroid is an elongated 
sphere. 

This figure, like an Oblate Spheroid, admits of various degrees of departure from the 
spherical form. It may be much or but slightly elongated, and the ends may be alike or 
otherwise. A common eg§ is an Oblong Spheroid. 




The Axis of a sphere is 
the line, real or imaginary, 
around which it revolves. 



AXIS OF A SPHERE. 



AXIS 




The Poles of a sphere 
are the extremities of its 
axis, or the points where 
the axis cuts the two op- 
posite surfaces. 

The Equator of a sphere is an imaginary circle upon 
its surface, midway between its poles, the plane of which 
cuts the axis perpendicularly, and divides the sphere 
into two equal parts or hemispheres. 



Kinds of spheroids ? Describe each. What is the axis of a sphere ? What 
the -jyoles ? The equator ? By what other name called ? What a Less Circle ? 
Meridians ? 



DEFINITIONS. 



19 



GKEAT AND LESS CIRCLES. 




MERIDIAN. 



The equator of a sphere is sometimes 
called a Great Circle, because no larger 
circle can be drawn upon its surface. 

A Less Circle is one that divides a 
sphere into two unequal parts. 

In the cut, the circles are represented in perspective. The 
Great Circle embraces the middle of the sphere, where its full 
diameter is included ; while the Less Circle passes around it 
between the Equator and the Poles, and is consequently " less' 1 
than the Equator. 

Meridians of a sphere are lines 
drawn from pole to pole upon its 
surface. 

16. Likes and Angles. 



A Point is that which has no magnitude or extension, 
but simply position. 

" The common notion of a point is derived from the extremity of some slender body, 
such as the extremity of a common sewing-needle. This being perceptible to the 
senses, is a physical point, and not a mathematical point ; for, by the definition, a 
point has no magnitude. 11 — Professor Perkins. 




A Right Line is the shortest distance 
between two points. 

A Curve Line is one that departs con- 
tinually from a direct course. 

Parallel Lines are such as remain at 
the same distance from each other through- 
out their whole extent. 

Oblique Lines are such as are not paral- 
lel, but incline toward or approach each 
other. 

"When two lines intersect or cut each 
other, the space included between them is 
called an Angle. 



A RIGHT LINE. 



CURVE LINE. 



PARALLEL LINES. 



OBLIQUE LINES. 



AN ANGLE. 




16. What is a point ? (Physical ? Mathematical ?) A right line ?— a curve 
line ?— parallel lines ?— an angle ?— kinds of angles ? Describe a right angle 
— an acute — an obtuse. 



20 



ASTRONOMY. 



EIGHT ANGLES. 



ACUTE AND OBTUSE 
ANGLES. 




Angles are of three kinds— namely, the Right Angle, 
the Acute Angle, and the Obtuse Angle. 

Right Angles are formed when one 
right line intersects another perpendicu- 
larly, and the angles on each side are 
equal. 

An Acute angle is one that is less, and 
an Obtuse angle one that is greater, than 
a right angle. 

17. Of Triangles. 

A Triangle is a plane figure, bounded by straight 
lines, and having only three sides. 

Triangles are of six kinds — viz., Right-angled, Obtuse- 
angled, Acute-angled, Equilateral, Isosceles, and Scalene. 

A Right-angled Triangle is one having 
one right angle. 

The parts of a Eight-angled Triangle 
are the Base, the Perpendicular, and the 
Hypothenuse. 

Hypothenuse, from a Greek word, which signifies to subtend or stretch — a line sub- 
tended from the base to the perpendicular. 

OBTUSE-ANGLED TRIANGLE. 

An Obtuse-angled Triangle is 
one having an obtuse angle. 



EIGHT-ANGLED 
TRIANGLE. 





ACUTE- ANGLED TRIANGLE. 



An. Acute-angled Triangle is one 
having three acute angles- 



An Equilateral Triangle has all three of 
its sides equal. 

Equilateral, from the Latin aquus, equal, and lateralis, from 
latus, side. 




AN EQUILATERAL 
TRIANGLE. 




17. What is a triangle ? How many kinds ? Describe (or draw) a right- 
angled triangle. Describe its parts. (Hypothenuse ?) An obtuse ? Acute ? 



DEFINITIONS. 



21 



ISOSCELES 
TRIANGLE. 




An Isosceles Triangle has only two of its 
sides equal. 

The term Isosceles is from a Greek word, signifying equal legs ; 
hence a triangle with two equal legs is called an Isosceles Triangle. 



A Scalene Triangle is one having no two sides 
equal. 

The term Scalene is from the Greek skalenos, and signifies oblique, unequal. (See 
obtuse and acute angled.) 

A CIRCLE. 

18. Circles and Ellipses. 

A Circle is a plane figure, bounded by a 
curve line, every part of which is equally 
distant from a point within called the center. 

Concentric Circles are such as are drawn 
around a common center. 

The Circumference of a circle is the curve 
line which bounds it. 




CONCENTRIC 
CIRCLES. 




DIAMETER, CIRCUMFER- 
ENCE, ETC. 




The Diameter of a circle is a right 
line passing through its center, and ter- 
minating each way in the circumfer- 
ence. 

The Radius of a circle is a right line 
drawn fom its center to any point in the 
circumference. 



The plural of radius is radii; and as radii proceed from a common center, light, 
which proceeds from a luminous point in all directions, is said to radiate ; and the 
light thus dispersed is sometimes called radiations or radiance. 

All circles, whether great or small, are supposed to be 
divided into 360 equal parts, called degrees ; each degree 
into 60 equal parts, called minutes ; and each minute 
into 60 equal parts, called seconds. They are marked 
respectively thus: Degrees (°), minutes ('), seconds ("). 

Equilateral ? (Derivation ?) Isosceles ? (Derivation ?) Scalene ? (Deriva- 
tion ?) 

IS. What is a circle ? Concentric circles ? The Circumference ? Diameter ? 
Radius ? (Plural, &c. ?) How all circles divided ? (What is a protractor? 



22 



ASTRONOMY. 



A PROTRACTOR. 




PARTS OF A CIRCLE. 

j g s 



To save the trouble of dividing a circle into 360°, in order to measure the degrees 
of an angle, we make use of an instrument called a Protractor. It consists of a semi- 
circle of silver or brass, divided into de- 
grees, as represented in the inclosed figure. 
To measure an angle, as ABC, the 
straight edge of the protractor is placed 
upon the line B C, so that the center 
around which it is drawn will be exactly 
at the intersection of the lines, or point of 
the angle, as at B ; then the number of de- 
grees included between the lines on the 
protractor will represent the quantity or 
amount of the angle. From this it will be 
seen that the amount of the angle does not 
depend upon the length of the lines which 
form it, nor upon the magnitude of the 

circle on which the degrees are marked by which it is measured, but simply upon the 
width of the opening^between the lines, as compared with the whole circumference 
around the point B. A circle marked off into degrees, minutes, and seconds, is called a 
graduated circle. 

Circles are also divided into Semicircles, Quadrants, 
Sextants, Signs, and Arcs. 

A Semicircle is the half of a cir- 
cle, or "180°. 

A Quadrant is one quarter of a 
circle, or 90°. 

The term Quadrant is applied to a nautical instru- 
ment, of the form of a quarter of a circle, which is much 
used by navigators in determining the altitude or appa- 
rent hight of the sun, moon, and stars. 

A Sextant is the sixth part of a 
circle, and contains 60°. 

The word Sextant also denotes an instrument similar to a Quadrant, and is used for 
similar purposes. The main difference is, that one represents G0°, and the other 90°, 
of a circle. The Octant, or eighth part of a circle, is also used for similar purposes. 

A Sign is the twelfth part of a circle, or 30°. 
An Arc is any indefinite portion of a circle. 

The word Arc is from the Latin arcus, a bow, vault, or arch. By associating the word 
arc with arch, the student may always remember its meaning. 

A Chord is a right line, joining the 
extremities of an arc. 

The Chord of an Arc is said to be subtended (from sub, 
under, and teno, to stretch), because it seems stretched under 
the arc like the string of a bow. In the cut, there are four 
arcs, and as many chords. The lower arc is a large one, 
while the arc and chord, A C, are quite small. Still each 
division of the circle, whether great or small, is an arc, and 
the line joining the extremities of each arc, respectively, is a 
chord. 




ARC AND CHORD. 




Describe. A graduated circle ?) What larger divisions of a circle ? What is 
a semicircle ? A quadrant ? (Note.) A sextant ? (Note.) A sign ? An 
arc ? (Derivation of term ?) Define a chord. (Why said to be subtended ?) 



DEFINITIONS. 



23 




MAJOR AND MINOR aXES. 




ECCENTRICITY OF AN ELLIPSE. 




An Ellipse is an oblong figure 
like an oblique view of a circle, 
having two points called its foci, 
around which, as centers, the figure 
is described. 

Foci is the plural of focus. 

The longer diameter of an ellipse 
is called its Major Axis, and the 
shorter its Minor Axis. 

Axes is the plural of axis. The longer is some- 
times called the Transverse, and the shorter the 
Conjugate, Axis ; but major and minor are more sim- 
ple and perspicuous, and therefore preferable. 

The Eccentricity of an ellipse 
is the distance between its cen- 
ter and either foci. 

Eccentric — ex, from, and centrum, center. 
Hence a circle that varies in its distance from the 
center is eccentric. So, also, persons who depart 
from the usual round of thought and custom are 
called eccentric persons. 

19. The Terrestrial Sphere. 

The Terrestrial Sphere is the earth or globe we in- 
habit. 

1. Though the earth is not, strictly speaking, a sphere, as that figure is defined (14), 
but rather an oblate spheroid (14), still it is usually called a sphere, on account of its 
near approach to that figure, and as a matter of con renience. 

2. Terrestrial, Latin" terrestris, from terra, the earth. "There are also celestial 
bodies, and bodies terrestrial ; but the glory of the celestial is one, and the glory of the 
terrestrial is another.'' — 1 Cor. xv. 40. 

The Axis of the earth is the imaginary line about which 
it makes its daily revolution. 

The Poles of the earth are the extremities of her axis 
where they cut or pass through the earth's surface. 

The wire upon which an artificial globe turns represents the earth's axis, and the 
extremities the North and South Poles. 

The Equator of the earth is an imaginary circle drawn 
around it, from east to west, at an equal distance from 
each Pole, and dividing it into two equal parts, called 
Hemispheres. 

See illustration, page 18. 



An ellipse? Its foci? (Plural and singular?) Major and minor axes? 
(Singular and plural ?) Eccentricity of an ellipse ? (Derivation ?) 

197 The terrestrial sphere ? (Is the earth a sphere ? Derivation of term 
terrestrial ?) Axis of the earth ? Poles ? Equator ? Latitude ? Parallels } 



24 



ASTRONOMY. 



Latitude upon the earth is distance either North or 
South of the Equator, and is reckoned each way toward 
the Poles in Degrees, Minutes, and Seconds. 

As the distance from the Equator to the Pole cannot be more than a quarter of a 
circle, or 90°, it is obvious that no place can have more than 90° of latitude ; or, in 
other words, all places upon the earth's surface must be between the Equator and 90° 
of latitude, either north or south. 



PARALLELS. 




Parallels of Latitude are circles 
either North or South of the Equator, 
and running parallel to it. 

We may imagine any conceivable number of parallels 
between the Equator and the Poles, though upon most 
maps and globes they are drawn only once for every ten 



The Tropics are two parallels of 
latitude, each 23° 28' from the 
Equator. 

The Northern is called the Tropic 
of Cancer, and the Southern the 
Tropic of Capricorn. 

1. The Tropical Circles are shown at E E in the an- 
nexed figure. 

2. The sun never shines perpendicularly upon any 
points on the earth further from the Equator than the 

Tropics. Between these he seems to travel regularly, leaving the Southern Tropic on 
the 23d of December, crossing the Equator northward on the 20th of March, reaching 
the Northern Tropic on the 21st of June, crossing the Equator southward on the 23d of 
September, and reaching the Southern Tropic again on the 23d of December. In this 
manner he seems to cross and recross the Equator, and vibrate between the Tropics 
from year to year. The cause of this apparent motion of the sun will be explained 
hereafter. 

The Polar Circles are two parallels of latitude, 23° 28 ; 
from the Poles. (See F F in the last cut.) 

The Northern is called the Arctic, and the Southern 
the Antarctic, Circle. 

The Tropics and Polar Circles divide the globe into 
five principal parts, called Zones, namely, one Torrid, 
two Temperate, and two Frigid. 

A zone properly signifies a girdle ; but the term is here used in an accommodated 
sense, as only three of these five divisions at all resemble a girdle. The parts cut off 
by the polar circles are mere convex segments of the earth's surface. 



THE 


TROPICS AND POLAB 




CIRCLE. 






E^-n 


1 — "s? 




P/ 


\ EC 




VF 


A ' 


IS 


AXJIS / 


\ A 




; < 








.* Z3 








; ^ 


'. \ 






/ ^ 


\ 





The tropics ? Names ? Polar circles ? Names ? Zones ? Names ? (Are 
there in reality any frigid zones f) Situation of the several zones ? Merid- 
ians ? Longitude <5n the earth ? First meridian ? (European and Ameri- 
can charts and globes ?) How longitude reckoned ? Its greatest extent ? 



DEFINITIONS. 



25 



THE FIVE ZONES. 




MERIDIANS. 




given 



The Torrid Zone is situated between 
the Tropics ; the Temperate, between 
the Tropics and the Polar Circles ; and 
the Frigid, between the Polar Circles 
and the Poles. 

Meridians are imaginary lines drawn 
from pole to pole over the earth's sur- 
face. 

Meridians cross the Equator at right angles ; and the 
plane of any two Meridians directly opposite each 
other -would divide the earth into Eastern and Western 
Hemispheres, as the Equator divides it into Northern 
and Southern. We may imagine Meridians to pass 
through every conceivable point upon the earth's sur- 
face. They meet at the Poles, and are furthest apart 
at the Equator. 

Longitude upon the earth is dis- 
tance either East or West of any 
meridian. 

A degree of longitude at the Equator comprises about 69| miles, but is less and 
less as the meridians approach the Poles, at which points it is nothing. A degree of 
latitude is about 69£ miles on all parts of the globe. 

The First Meridian is that from which the reckoning 
of Longitude is commenced. 

On European charts and globes, longitude is usually reckoned from the Eoyal Ob- 
servatory at Greenwich, near London ; but in this country it is often reckoned from 
the Meridian of Washington. It would be better for science, however, if all nations 
reckoned longitude from the same Meridian, and all charts and globes were constructed 
accordingly. 

As Longitude is 
reckoned both East 
and West, the great- 
est longitude that 
any place can have 
is 180°. 

20. The Celestial 
Sphere. - 

The Celestial 
Sphere is the appa- 
rent concave sur- 
face of the hea- 
vens, surrounding 
the earth in all di- 
rections. 

The relation of the Terrestrial to the Celestial Sphere may be understood by the 
ubove diagram, in which the stars surround the earth in all directions, as they seem to 
fill che whole celestial vault. 

9 



TERRESTRIAL AND CELESTIAL SPHERES. 




26 



ASTRONOMY. 




The Axis of the Heavens is the axis of the earth pro- 
duced or extended both ways to the concave surface of 
the heavens. 

The EaUdtOT of the equator of the heayens, oe equinoctial. 

heavens, or Equinoc- 
tial, is the plane of 
the Earth's equator 
extended to the starry 
heavens. 

Declination is dis- 
stance either north or 
south of the Equinoc- 
tial. 

Declination is to the heavens 
precisely what latitude is upon 
the earth. It is reckoned from 
the celestial equator, both North 
and South, to 90°, or to the poles 
of the heavens. Celestial Lati- 
tude can be explained better 
hereafter, and so with the terms 
Ecliptic, Zodiac, &c. 

Right Ascension is distance east of a given point, and 
is reckoned on the Equinoctial quite around the heavens. 

In one respect, Eight Ascension in the heavens is like longitude on the earth: 
they are both reckoned upon the equators of their respective spheres. But while 
■ ^iigitude is reckoned both east and west of the first meridian, andean only amount to 
180°, Eight Ascension is reckoned only eastward, and consequently may amount to 
860°, or the whole circle of the heavens. The principal difference between Eight As- 
cension and Celestial Longitude is, that the former is reckoned on the Equinoctial, and 
the latter on the Ecliptic. 

The Sensible Horizon is that 
circle which terminates our view, 
or where the earth and sky seem 
to meet. 

The Hational Horizon is an 
imaginary plane, below the visible 
horizon, and parallel to it, which, 
passing through the earth's cen- 
ter, divides it into upper and lower hemispheres. 

1. These hemispheres are distinguished as upper and lower with reference to the ob- 
server only. 

20. Celestial sphere ? (Relation to terrestrial ?) Axis of the heavens ? 
Equator of the heavens ? Declination ? (How illustrated by terrestrial 
latitude ? How reckoned ? Its limits ?.) Right ascension ? (How resemble 
longitude ? What difference ?) Sensible horizon ? Rational ? Explain by 



SENSIBLE 



RIZON 







DEFINITIONS. 



27 



2. The sensible horizon is half the diameter of the earth, or about 4,000 miles from 
the rational ; and yet so distant are the stars, that both these planes seem to eut the 
celestial arch at the same point: and we sec the same hemisphere of stars above the 
sensible horizon of any place that we should if the upper half of the earth were re- 
moved, and we stood on the rational horizon of that place. 

The Poles of the Horizon are two opposite points — 
one directly above, and the other directly beneath, 
us. The first is called the Zenith, and the latter the 
Nadir. 

The points Up and Down, East and West, are not 
positive and permanent directions, but merely rela- 
tive. 



FP AND DOWN, AND EAST AND 
WEST. 




*H~ 



»- 



1. As the earth is a sphere, inhabited on all sides, 
the Zenith point is merely opposite its center^ and the 
Nadir toicard its center. So with the directions Up 
and Dozen: one is from the center, and the other 
toicard it ; and the same direction which is up to one, 
is doicn to another. This fact should not merely be 
acknowledged, but should be dwelt upon until the mind 
has become familiarized to the conception of it, and di- 
vested, as far as possible, of the notion of an absolute 
up and down in space. We should remember that we 
are bound to the earth's surface by attraction, as so 
many needles would be bound to the surface of a spher- 
ical loadstone. 

2. East and West also are not absolute, but merely 
relative, directions. East is that direction in which 
the sun appears to rise, and West is the opposite direc- 
tion ; and yet, so far as absolute direction is concerned, 
what is East to one, as to the observer at A, is West to 

B, and so with C and D. And as the earth revolves upon its axis every twenty-four 
hours, it is obvious that East and West upon its surface must, in that time, change to 
every point in the whole circle of the heavens. The same is true of the Zenith and 
Nadir, or of up and down. 

ISjMce, in Astronomy, is that boundless interval or void 
in which the earth and the heavenly bodies are situated, 
and extending infinitely beyond them all, in every direc- 
tion. 

Space has no limits — or, in other words, is boundless, or infinite. Suppose six 
persons were to start from as many different points upon the earth's surface — as, for 
instance, one from each pole, and one from each of the positions occupied by observers 
in the next figure. Let them ascend or diverge from the earth in straight lines, perpen- 
dicularly, to its surface, and though they were to proceed onward," separating from 
each other, with the speed of lightning, for millions of ages, none of these celestial 
voyagers would find an end to space, or anj^ effectual barrier to hinder their advance- 
ment Should they chance to meet another world in the line of their flight, it would 
soon be passed, like a ship met by a mariner upon the ocean, and beyond it space 
would stiil invite them onward to explore its immeasurable depths. And thus they 
might go on forever, without changing their position in respect to the center or boun- 
daries of immensity ; for as eternity has no beginning, middle, or end, so space is with- 
out center or circumference — an ethereal ocean, without bottom or shore. 



diagram. Poles of- the horizon ? Names? Up and down — positive or rela- 
tive points? (Illustrate by diagram; also east and west.) Term space in 
astronomy? (Has it any limits \ Illustration.) 



28 



ASTRONOMY. 



TITE SOLAK SYSTEM. 




SOLAR SYSTEM AND SIDEREAL HEAVENS. 



21. First Grand 
Divisions of the 
Universe. 

The visible uni- 
verse may be con- 
sidered under two 
grand divisions — 
viz., the Solar Sys- 
tem and the Side- 
real Heavens. 

The Solar System 
consists of the sun 
and all the planets 
and comets that re- 
volve around him. 

The Sidereal Ilea- 
vens include all 
those bodies that lie 
around and beyond 
the Solar System, 
in the region of the 
Fixed Stars. 

1. The word Sidereal is 
from the Latin sideralis, and 
signifies pertaining to the 
stars. The Sidereal Heavens 
are, therefore, the heavens of 
the fixed stars. 

2. The relation of the Solar 
System to the Sidereal Hea- 
vens is shown in the annexed 
cut, where the sun appears 
only as a star, at a distance 
from all others.and surrounded 
by his own retinue of worlds. 
The Solar System is drawn 
upon a small scale, and the 
Sidereal Heavens are seen 
around and at a distance from 
it in every direction. 

In considering the general subject of Astronomy, we 
shall proceed according to the foregoing classification, 
treating first of the Solar System, and, secondly, of the 
Sidereal Heavens. 




21. How visible universe divided? Define each? (Derivation of term 
sidereal ? Relation of solar system to the sidereal heavens ? Illustrate by 
drawing.) Of which division does the author first treat ? 



PART I. 

THE SOLAR SYSTEM 



CHAPTER I. 

THE PRIMARY PLANETS. 

22. The Solar System derives its name from the Latin 
term sol, the sun. It signifies, therefore, the System of 
the Sun. It includes that great luminary, and all the 
planets and comets that revolve around him. 

23. The Sun is the fixed center of the system, around 
which all the solar bodies revolve, and from which they 
receive their light and heat. 

24. The Planets are those spherical bodies or worlds 
that revolve statedly around the sun, and receive their 
light and heat from him. 

The term planet signifies a wanderer, and was applied to the solar bodies because 
they seemed to move or wander about among the stars. 

The Orbit of a planet is the path it pursues in its revo- 
lution around the sun. 

25. The planets are divided into Primary and Secon- 
dary planets. 

The Primary Planets are those larger bodies of the 
system that revolve around the sun only, as their center 
of motion. 

The Secondary Planets are a smaller class of bodies, 

22. Of what does Part II. treat ? What meant by the Solar System ? I_a 
chides what ? 

23. What is the sun ? 

24. Describe the planets. (The term ?) The orbit of a planet ? 

25. How planets divided ? Describe each. (What other names for secon- 
daries ?) 



so 



ASTRONOMY. 



that revolve not only around the sun, but also around the 
primary planets, as their attendants, or moons. 

The secondary planets are also called Moons or /Satellites. A satellite is a follower 01 
attendant upon another. 



VIEW OF THE SOLAR SYSTEM. 




In this cut, the sun may be seen in the center. The white, circles are the Orbits 
of the primary planets. "^The planets may be seen in those orbits at various distances 
from the sun. The numerous orbits so close together arc those of the Asteroids. The 
secondary planets may be seen near their respective primaries, revolving around them, 
while they all go on together around the sun. On the right is seen a Comet plunging 
into the system, with his long and fiery train. His orbit is seen to be very elliptical. 
All these bodies are opake, the sun excepted. Even the blazing comet shines only by 
reflection. 

26. The planets are again divided into Interior and 
Exterior planets. 

The Interior Planets are those whose orbits lie icithin 
the orbit of the earth, or between it and the sun. 

26. What meant by interior and exterior planets ? (Why not inferior and 
superior ?) 






PRIMARY PLANETS. 



31 



The Exterior Planets are those whose orbits lie with- 
out the orbit of the earth. 

Some Astronomers speak of these two classes respectively as Inferior and Superior. 
The reason seems to be, that as those nearer the sun than the earth are lower than she 
is — that is, nearer the great center of the system — they are, in this respect, inferior 
to her; white, on the other hand, those that are above, or* beyond her, are her superiors. 
But as the distinction is founded upon, and is intended to denote, the position of the 
planets with respect to the earth's orbit, it is obvious that interior and exterior are the 
more appropriate terms. It seems hardly allowable to call the Asteroids superior plan- 
ets, and Mercury and Venus, which are much larger, inferior. 

27. Fifteen of the smaller primary planets are called 
Asteroids. 

Asteroid signifies star-like, and is applied to these small planets because of their 
comparative minuteness. They are never seen except through telescopes, and through 
ordinary instruments are not always readily distinguished from the fixed stars. 

28. Comets are a singular class of objects, belonging 
to the solar system, distinguished for their long trains of 
light, their various shapes, and the great eccentricity of 
their orbits. 

NUMBER AND NAMES OF THE PRIMARY PLANETS. 

29. The Primary Planets are twenty-three in num- 
ber. They are denoted, in astronomical works, by cer- 
tain characters, or symbols : and as the names of the 
planets are mostly derived from Mythology, their sym- 
bols generally relate to the imaginary divinity after whom 
the planet is named. The names of the planets and 
their symbols are as follows : 

Mercury £ 



Venus ? 

Earth e 

Mars $ 

f Flora %$. 

Clio ^ 

Vesta 

Iris 

<( Metis 

Hebe g 

Parthenope <& 

Egeria* 

^ Astnea ^ 



S 



f Irene ^ 

Eunomia 

Juno o 

Ceres 9 

<; Pallas * 

Hygeia 

Melpomene 

Misillia 

Anonymous 

Jupiter u 

Saturn ^ 

Uranus ig 

jSTeptune % 



* For some of these new planets, no sign has yet been determined upon. 

27. What are the asteroids ? How many? (Term? Are they visible to 
the naked eye ?) 

28. What are comets ? (Describe the preceding cut. Where sun ? Prim- 



32 



ASTRONOMY. 



1. The planets are placed in the order of their distances, respectively, beginning at 
the sun. 

2. We have not in every case been able to procure the astronomical symbol. This 
accounts for the blanks opposite several of the names. 

3. The names of the eighteen asteroids are included in braces. 



MYTHOLOGICAL HISTORY AND SYMBOLS. 



30. 



gods. 



messenger of the 



ROD OF MERCURY. 




MIRROR OF VENUS. 



Mercury was the 
s, and the patron of thieves and dishon- 
est persons. His symbol denotes his cadu- 
ceus, or rod, with serpents twined around 
it (3).* 

1. Mercury was represented as very eloquent, and skillful in in- 
terpreting and explaining — as the god of rhetoricians and orators. 
Hence, when Paul and Barnabas visited Lystra, addressed the peo- 
ple, and wrought a miracle, they said, " The gods have come down 
to us in the likeness of men. And they called Barnabas Jupiter, 
and Paul JSIercurius, because he was the chief speaker" 

2. " The caduceus of Mercury was a sort of wand or scepter, borne by Mercury as an 
ensign of quality and office. On medals, it is a symbol of good conduct, peace, and 
prosperity. The rod represents poicer ; the serpents, wisdom; and the two wings, 
diligence and activity."" — Encyclopaedia. 

3. The original form of this sign may be understood by the preceding cut, to which the 
present astronomical symbol ( £ ) bears but a slight resemblance. 

31. Venus was the goddess of love 
and beauty, and her sign is an ancient 
mirror or looking-glass ( ? ), which she is 
represented as carrying in her hand. 

Anciently, mirrors were made of brass or silver, highly pol- 
ished, so as to reflect the image of whatever was brought before 
them. Hence it is said in the Book of Exodus, written fifteen 
centuries before Christ, that Moses " made the laver of brass, 
and the foot of it of brass, of the looking-glasses of the women,*' 
&c. For convenience, the ancient mirrors had a handle at- 
tached, as represented in the cut, which very much resembles 
the sign of the planet. 

32. The Earth (called by the Greeks 
Ge, and by the Latins Terra) has two sym- 
bols — one representing a sphere and its equator (e), and 
the other (0) the four Quarters of the globe. 

* All these symbols should be drawn in rotation upon the Blackboard, during recita- 
tion, by the Teacher, or some member of the class. It will be well, therefore, for the 
student to observe each sign carefully, that he may be prepared to draw and explain 
it, if called upon. 

aries ? Secondaries ? Asteroids ? Orbits ? Comet and orbit ? Which, 
self-luminous, and which opake ?) 

29. How many primary planets ? How represented in astronomical works ? 
Origin of names and symbols ? Repeat names. Draw symbols on black- 
board. (In what order arranged ? How asteroids designated ?) 

30. Who was Mercury, in Mythology, and what does his symbol denote ? 
(How was he represented ? What Scriptural allusion? Describe his cadu- 
ceus. The meaning of its parts ?) 




MYTHOLOGICAL HISTORY AND SYMBOLS. 33 




33. Mars was the god of war, and his sign 

($) represents an ancient shield or buckler. 8PBAR AND 8HIELD 

N y -L 7 OF MARS. 

crossed by a spear. 

Gunpowder was not known to the ancients, consequently they 
had no pistols, muskets, or cannon. They fought with short swords 
and spears, and defended themselves with the shield, carried on 
the left arm. A shield and spear were, therefore, very appropriate 
emblems of war. The original form of the sign of Mars is pre- 
sented in the cut. 

34. Flora was the " queen of all the 
flowers," and her symbol (*$) is a flower, 
the "Kose of England." 

35. Clio was one of the Muses. Her sign ("#) is a 
star, with a sprig of laurel over it. 

36. Vesta was the goddess of fire, and her sign (£) is 
an altar, with a fire blazing upon it. 

37. Iris w^as the beautiful waiting-maid of Juno, the 
queen of heaven. Her symbol (&s) is composed of a 
semicircle, representing the rainbow, with an interior 
star, and a base line for the horizon. 

"As an attendant upon Juno,''' says Prof. Hind, "the name was not inappropriate at 
the time of discovery, when Juno was traversing the ISth hour of right ascension, and 
was followed by Iris in the 19th. 1 ' 

38. Metis was the first wife of Jupiter, and the god- 
dess of prudence and sagacity. Her symbol (&) is # an 
eye (denoting wisdom) and a star. 

39. Hebe presided over children and youth, and was 
cnp-bearer to Jupiter. Her sign ( 2 ) is a cup. 

Hebe was celebrated for her beauty, but happening one day to stumble and spill the 
nectar, as she was serving Jupiter, she was turned into an ostler, and doomed to harness 
and drive the peacocks of the queen of heaven. 

40. Parthenope was one of the three Syrens— & sea 
nymph of rare beauty. They were all admirable singers; 
hence a lyre (<f) is her appropriate sign. 

1. The three Syrens — Parthenope, Ligeia, and Leucosia — were represented as dwell- 

31. Venus and symbol ? (Ancient mirrors ? Scripture allusion ?) 

32. The Earth — ancient name and symbols ? 

33. Mars and symbol ? (Ancient mode of warfare ?) 

34. Flora and sign ? 

35. Clio and symbol ? 

36. Vesta and her symbol ? 

37. Iris and her sign ? (Prof. Hind's remark ?) 

38. Metis and her sign ? 

39. Hebe and her sign ? ('Incident mentioned in note ?) 

40. Parthenope and sign ? (What said of the Syrens ? Of the appro 
priateness of the name ?) 

2* 



34 ASTRONOMY. 



ing upon the coast of Sicily, and luring mariners upon the rocks of destruction by their 
enchanting songs. Hence whatever tends to entice or seduce to ruin is often called a 
" syren song/' 

2. As this planet was discovered at the Naples Observatory, in Italy, it was quite ap- 
propriate to name it after one of the Syrens, that Mythology located on the coast of a 
neighboring island. 

41. Egeria was the counsellor of Numa Pompilius. 
Symbol not yet agreed upon by astronomers. 

42. Astr^a was the goddess of Justice, and her sign 
(&) is a balance. 

Mythology teaches that Justice left heaven, during the golden age, to reside on the 
earth; but" becoming weary with the iniquities of men, she returned to heaven, and 
commenced a constellation of stars. The constellations Virgo and Libra in the zodiac 
are representations of Astra?a and her golden scales. So the female figure, holding a 
pair of scales, in the coat, of arms of several of the United States, is a representation of 
Astraea, and denotes Justice, 

43. Irene was one of the Seasons. The planet was so 
named by Sir John Herschel, in honor of the peace pre- 
vailing in Europe at the time of its discovery (May, 
1851). Its symbol (ak) is a dove, with an olive branch in 
her mouth, and a star upon her head. 

44. Eunomia was another of the Seasons — a sister of 
Irene. (Symbol not ascertained.) 

45. Juno was the reputed queen of heaven, and her 
sign (5) is an ancient mirror, crowned with a star — an 
emblem of beauty and power. 

46. Ceres was the goddess of grain and harvests, and 
her sign (?) is a sickle. 

47. Pallas (or Minerva) was the goddess of wisdom 
and of war. Her symbol ( $ ) is the head of a spear. 

1, The ancient Palladium -was an image of Pallas, preserved in the castle of the city 
of Troy ; for while the castle of the city of Minerva was building, they say this image 
fell from heaven into it, before it was covered with a roof. — Tooke's Pantheon. 

2. To a similar fable, respecting an image falling from heaven, the Apostle Paul al- 
ludes, Acts xix. 35: — "Ye men of Ephesus, what man is there that knoweth not how 
that the city of Ephesus is a worshiper of the great goddess Diana, and of the imago 
which fell down from Jupiter V 

41. Egeria and her symbol ? 

42. Astraea and sign ) (Mythological legend ? Virgo and Libra ? "Where 
else found ?) 

43. Irene — by whom named, and why ? Symbol ? 

44. Eunomia and symbol? 

45. Juno and symbol ? 

46. Ceres and her symbol? 

47. Pallas and her" symbol ? (Ancient Palladium ? Keputed origin i 
Scriptural allusion to it ?) 



MYTHOLOGICAL HISTORY AND SYMBOLS. 



35 



SATURN, OR CHRONOS. 



48. Hygeia was the goddess of health, and the daugh- 
ter of Esculapius, the father of the healing art. (Symbol 
not ascertained.) 

Our modern vrord Ilygeian, which signifies the laws of health, &c, is derived from 
the goddess Hygeia. 

49. Jupiter was the reputed father of the gods — the 
king of heaven. His symbol (U) was originally the 
Greek letter £, zeta (the same as our Z) — the initial of 
the Greek word zeus, the name for Jupiter. 

50. Saturn — called by the 
Greeks Chronos — presided 
over time and chronology. 
His sign (*?) represents a 
scythe. 

1. Saturn was represented in Mythology as 
an old man, with wings, bald excepting aibre- 
lock, with a scythe in one hand, and an hour- 
glass in the other. The same figure is now 
used to represent time. 

2. Our modern word chronology, from 
chronos, time, and logos, discourse, signifies 
the science of keeping time, dates, &c. 

51. Uranus was the father 
of Saturn, and presided over 
astronomy. The symbol of 
this planet ( r #) consists of the 
letter H, with a planet suspended from the cross-bar, in 
honor of Sir William Herschel, its discoverer. 

This planet is popularly known by the name of Herschel, but astronomers now almost 
universally call it Uranus. It bears this name in the British Nautical Almanac for 
1S51, with the full consent of Sir John Herschel, the son of the great discoverer. It was 
first called Georgium Sidus, by Dr. Herschel, in honor of his royal patron, George III. 

52. ISTeptune was the god of the seas, but the symbol 
of the planet (#) is composed of an L and a V united, 
with a planet suspended from the hair-line of the V, in 
honor of Le Verrier, its discoverer. 

This planet was first called Le Terrier, but is more generally known by the name of 
Neptune. 

53. The Moon was called Luna bv the Romans, and 




43. Hygeia and symbol ? (Term hygeian?) 

49. Jupiter and his symbol ? 

50. Saturn ? Greek name ? Symbol ? (How represented in Mythology? 
Word chronology ?) 

51. Uranus and symbol ? (What other names, and why?) 

52. Neptune and his symbol ? (Former name ?) 



36 



ASTRONOMY. 



Selene by the Greeks. She is known by various sym- 
bols, according as she is new, half-grown, or full, 
thus: •, «D, O. 

1. From Luna we have our modern terms lunar and lunacy ; the former of which 
signifies pertaining to the moon, and the latter a disease anciently supposed to be caused 
by the moon. 

2. Selene, in Mythology, was the daughter of Helios, the Sun. Our English word 
selenography — a description of the moon's surface — is from Selene, her ancient name, 
and grapho, to describe. 

54. The Sun — called Sol by the Romans, and Helios 
by the Greeks — is represented by a shield or buckler, 
thus : ©» ©> ®. As the large and polished bucklers of 
the ancients dazzled the eyes of their enemies, this in- 
strument was selected as an appropriate emblem of the 
sun. 



DISTANCES OF THE PLANETS. 

55. The distances of the planets from the sun, com- 
mencing with Mercury, and proceeding outward, are as 
follows, viz. : 



Mercury ... 37 millions, 


or 36,890,000 


Venus .... 69 


u 


68,770,000 


Earth . . . . 95 


tc 


95,298,260 


Mars .... 145 


i. 


145,205,000 


The Asteroids, from 210 


cc 


to 300,000,000 


Jupiter .... 496 


u 


or 495,817,000 


Saturn .... 909 


u 


909,028,000 


Uranus .... 1,800 


u 


1,829,071,000 


Neptune . . . 2,862 


i< 


2,862,457,000 



1. The first column of round numbers only should be committed to memory by the 
student. These should be well fixed in the mind, as it will greatly facilitate the pro- 
gress of the student hereafter. The family of Asteroids being less important, their dis- 
tances need not be learned in detail. The following table shows the distances of the 
several Asteroids from the sun : 



Flora 209,826,000 

Clio 222,313,000 

Yesta 225,000,000 

Iris 227,334,000 

Metis 227,3S7,000 

Hebe 231,089,000 

Parthenope 233,611,000 

Egeria 244,940,000 



Astraea. 245,622,000 

Irene 246,070,000 

Eunomia 252,300,000 

Juno 254,312,000 

Ceres 263,713,000 

Pallas 264,256,000 

Hygeia 300,322,000 



53. The Moon — Latin and Greek names? Symbols? (Words lunar and 
lunacy t Who was Selene in Mythology ? Selenography ? Derivation ?) 

54. The Sun — Latin and Greek names ? Symbol, and why ? 

55. Kehearse, in round numbers, the distances of the planets from the 
(Substance of note 1st ? Object of note 2d ? Note 3d t Note 4th ?) 



DISTANCES OF THE PLANETS. 37 



2. The comparative distances of the planets are represented in the cut, page 15, and 
also in the following : 



COMPARATIVE DISTANCES OF THE PLANETS. 

^:x%, \ \ \ \ 

^ : ?'h%\ 3\ Si -, H N] 

T 



TLJJIi M ! I 



I 

3. To assist his conception of these vast distances, the student may imagine a rail- 
road laid down from the sun to the orbit of Neptune. Now if the train proceed from 
the sun at the rate of thirty miles an hour, without intermission, it will reach Mercury 
in 152 years; the Earth in 361 years; Jap iter in 1,884 years; Saturn in 3,493 years; 
Ilerschelin 6,933; and Neptune in 10,S00 years ! Such a journey would be equal to 
riding 900,000 times across the continent, from Boston to Oregon ! 

4. It is now about 5,S50 years since the creation of the world. Had a train of cars 
started from the sun at that time toward the orbit of Neptune, and traveled day and 
night ever since, it would still be 2S4 millions of miles within the orbit of Herschel — ■ 
about where the head of the locomotive stands, as shown in the cut! To reach even 
that planet would require over 1.000 years longer ; and to arrive at Neptune, nearly 
6,000 years to come ! Such is the vast area embraced within the orbits of the planets, 
and the spaces over which the sunlight travels, to warm and enlighten its attendant 
worlds. 

56. The apparent magnitude of the heavenly bodies 
depends much upon the distance from which they are 
viewed ; the magnitude increasing as the distance is 
diminished, and diminishing as the distance is increased. 



NEAR AND REMOTE VIEWS OF THE SAME OBJECT. 




/ 




r 


~7->t ~~TX-— --"-*'■* 




• {. 


n — ; _ .» 

:-"'-- \~' 




\ 



Let A represent the position of an observer upon the earth, to whom the sun appears 
82', or about half a degree in diameter. Now it is obvious that if the observer advance 
to B (half way), the object will fill an angle in his eye twice as large as it rilled when 
viewed from A. Again : if he recede from A to C, the object will appear but half as 
large. Hence the rule, that the apparent magnitude is increased as the distance is 
diminished, and diminished as the distance is increased. 

57. Could a beholder leave the earth, and, descending 
toward the sun, station himself upon Mercury, he would 
find the apparent magnitude of the sun vastly increased. 
Should he then return, and pass outward to Mars or 
Jupiter, he would observe a corresponding diminution in 
the sun's magnitude, in proportion as the distance was 
increased. Hence the apparent magnitude must vary 

56. How apparent magnitudes of heavenly bodies modified ? (Illustrate 
by diagram.) 

57. Suppose a person to go to Mercury — what effect upon apparent size of 
the sun ? 



38 



ASTRONOMY. 



exceedingly, as viewed from different points in the solar 
system. 

THE SUN, AS SEEN FROM THE DIFFERENT PLANETS. 



From 

N. H. S. Jupiter. Mars. 



.00 





The above cut represents the relative apparent magni- 
tude of the sun, as seen from the different planets. In 
angular measurements, its diameter would be as follows : 



From Mercury 


• 82J' 


From Jupiter 


. 6' 


" Venus . 


. 44i' 


" Saturn . . 


• 3i' 


" Earth . 


. 32' 


" Uranus 


. n r 


" Mars . 


. 21' 


" Neptune . 


. 50" 


The Asteroids, sa 


y 12' 







Let us continue our imaginary journey outward, be- 
yond Neptune, toward the fixed stars, and in a short 
time the glorious sun, so resplendent and dazzling to our 
view, will appear only as a sparkling star / and the fixed 
stars will expand to view as we approach them, till they 
assume all the magnitude and splendor of the sun him- 
self. 



LIGHT AND HEAT OF THE PLANETS. 

58. As the distances of the planets, respectively, affect 
the apparent magnitude of the sun, as viewed from their 
surfaces, so it must affect the relative amount of light 
and heat which they respectively receive from this great 
luminary. 

59. The amount of light and heat received from the 
sun, by the several planets, is in inverse proportion to 
the square of their respective distances. 

58. What effect has the distances of the planets from the sun, respectively, 
upon their relative light and heat ? 

59. W T hat rule governs the diffusion of light ? (Illustrate by a diagram.) 



LIGHT AND HEAT OF THE PLANETS. 



39 



PHILOSOPHY Or THE DIFFUSION OF LIGHT. 




1. Here the light is seen passing in right lines, from the sun on the left toward the 
several planets on the right It is also shown that the surfaces A, B, and C receive equal 
quantities of light, though Bis four times, and C nine times, as large as A ; and as the 
light falling upon A is spread over four times as much surface at 15, and nine times as 
much at G, it follows that it is only one-ninth as intense at C, and one-fourth at B, as it 
is at A. Hence the rule, that the light and heat of the planets is, inversely, as the 
squares of their respective distances. 

2. The student may not exactly understand this last statement. The square of any 
number is its product, when multiplied by itself. Now suppose we call the distances 
A, B, and C 1, 2, and 3 miles. Then the square of 1 is 1 ; the square of 2 is 4; and the 
square of 3 is 9. The light and heat, then, would be in inverse proportion at these 
three points, as 1, 4, and 9 ; that is, four times less atB than at A, and nine times less at 
C. These amounts we should state as 1, %, and 1 

60. The intensity of light and heat received upon the 
several planets varies, according to their respective dis- 
tances, from 6J times as much as our globe to 9 J^th part 
as much. 

1. The comparative light and heat of the planets — the earth being 1 — is as fol- 
lows : 



Mercury 6£ 

Venus 2 

The Earth 1 

Mars i 

The Asteroids I 



Jupiter 

Saturn 

Uranus yj, 

Neptune -^ 



t 

1 

9TT 



2. From this table it appears that Mercury has 6| times as much light and heat as our 
globe, Uranus only ^^, and Neptune only -p^th part as much. Now if the average 
temperature of the earth is 50 degrees, the average temperature of Mercury would be 
325 degrees ; and as water boils at 212, the temperature of Mercury must be 113 degrees 
above that of boiling water. Venus would have an average temperature of 100 degrees, 
which would be twice that of the earth. On the other hand, Jupiter, Saturn, Uranus, 
and Neptune, seem doomed to the rigors of perpetual winter. And what conception 
can we form of a region 900 times colder than our globe ! Surely, 

" Who there inhabit must have other powers, 
Juices, and veins, and sense, and life, than ours; 
One moment's cold, like theirs, would pierce the bone, 
Freeze the heart's blood, and turn us all to stone !" 

8. It is not certain, however, that the heat is proportionate to the light received by 



60. Between what limits does the light and heat of the several planets 
vary ? (What would that be for Mercury ? For Venus ? How with the ex 
tenor planets ? Poetry ? Is it certain that the heat of the planets is in exact 
proportion to the light they respectively receive ? Why not ?) 



40 



ASTRONOMY. 



the respective planets, as various local causes may conspire to modify either extreme of 
the high or low temperatures. For instance, Mercury may have an atmosphere that ar- 
rests the light, and screens the body of the planet from the insupportable rays of the 
sun; while the atmospheres of Saturn, Herschel, &c, may act as a refracting medium to 
gather the light for a great distance around them, and concentrate it upon their other- 
wise cold and dark bosoms. 



MAGNITUDE OF THE PLANETS. 

61. The planets vary as much in their respective mag- 
nitudes, as in their distances. Their several diameters, 
so far as known, are as follows : 



2,950 
7,900 
7,912 
4,500 



295 



Astrsea . . 


. . 


Irene . . . 


— 


Eunomia . 


— 


Juno . . 


. . 1,400 


Ceres . . . 


163 


Pallas . . . 


770 


Hygeia . . 
Jupiter . . 
Saturn . . , 


'. .' 88,790 
. 69,300 


Uranus . . 


. . 35,000 


Neptune . 


. . 31,000 



Mercury . . 
Venus . . . 
Earth . . . 
Mars . . . 
Flora . . . 
Clio .... 
Yesta . . . 
Iris .... 
Metis . . . 
Hebe . . . 
Parthenope . 
Egeria . . . 

1. The asteroids are so small and so remote, that measurements of their exact diam- 
eters are obtained with great difficulty ; hence the numerous blanks in the above table. 
And even when diameters are given, they are somewhat doubtful. 

2. In the case of the other planets, we have given their mean or average diameters, 
according to the best authorities. As most of them are more or less oblate, their polar 
diameters are less, and their equatorial more, than the amount given in the table. 

62. The magnitude of the principal planets, as com- 
pared with the earth, is as follows : — Mercury, T V as large; 
Venus, T 9 o ; Jupiter, 1,400 times as large ; Saturn, 1,000 
times ; Uranus, 90 times ; and Neptune, 60. 

1. The magnitudes of spherical bodies are to each other as the cubes of their diam- 
eters. Thu$r79 12 X 791 2 x7912=495,2S9, 174,428, the cube of the earth's diameter; and 
«950 X 2950X~950=25, 072,375,000, the cube of the diameter of Mercury. Divide the 
former by the latter, and we have 19 and a fraction as the number of times the bulk of 
Mercury is contained in the earth. 



61. State the diameters of the several planets ? (Why blanks in the 
table ? What diameters are given — polar, equatorial, or neither?) 

62. Give the magnitude of the principal planets, as compared with the 
earth. (How ascertain relative magnitudes ? Uow possible that a mere star 
can be such an immense world t) 



DENSITY 



41 



COMPARATIVE MAGNITUDK OF THE SUN AND PLANETS. 




2. It may seem almost incredible that what appear only as small stars in tho heavens 
should be larger than the mighty globe upon which we dwell. But when we consider 
their immense distance, and the effect this must have upon their apparent magnitude, 
as illustrated at 55, it is evident that the planets could not be seen at all were they not 
very large bodies. The above cut will give some idea of the magnitude of the several 
planets, as compared with each other, and also with the sun. 

63. The Sun is 1,400,000 times larger than our globe, 
and 500 times larger than all the other bodies of the solar 
system put together. It would take one hundred and 
twelve such worlds as our earth, if laid side by side, to 
reach across his vast diameter. 



DENSITY. 



64. The planets differ greatly in their density, or in the 
compactness of the substances of which they are com- 
posed. Mercury is about three times as dense as our 
globe, or equal to lead. Venus and Mars are about the 
same as the earth ; while Jupiter and Uranus are only jth 
as dense, or about equal to water. Saturn has only -^ th 
the density of our globe, answering pretty nearly to cork. 



63. State the magnitude of the sun as compared with the earth. With the 
rest of the system. Illustration 3 

64. What meant by density? Do the planets differ in this respect 3 State 
und illustrate. (How masses of planets ascertained ? How with Mercury I) 



42 



ASTRONOMY. 



ATTRACTION OF THE EARTH. 



The masses of the planets are determined by the revolution of their respective satel- 
lites ; but as Mercury has no satellite, the determination of his mass and density be- 
comes a very difficult aud uncertain matter. "But it fortunately happens," says Pro£ 
Hind, "that we have a curious method of approximating to this element, viz.," by the 
perturbations produced by the planet in the movements of a comet known as Euchis, 
which revolves around the sun in little more than three years, and occasionally ap- 
proaches very near Mercury, &c. From computations based" upon these perturbations, 
Prof. H. concludes that Mercury is only about y 1 ^- more dense than our globe — a result 
widely different from that arrived at by his predecessors. > 

GRAVITATION. 

65. Attraction, or Gravitation, is. the tendency oi 
bodies toward each other. It is that tendency which 
causes bodies raised from the earth, and left without sup- 
port, to fall to its surface. 

All substances fall toward the earth's 
center from every part of the globe, as a 
spherical loadstone would attract parti- 
cles of steel to its surface in every di- 
rection. Hence when these four men, 
standing on different sides of the globe, 
drop each a stone, they all fall toward 
the same point, because the earth at- 
tracts them all to herself. 

66. Gravitation is what 
constitutes the weight of 
bodies, and depends upon 
the quantity of matter in 
the bodies attracting, and 
their distances from each 
other. 

The reason why a cubic foot of cork weighs much less than the same bulk of lead, is, 
that being less dense, it contains much less matter to be attracted. 

67. From the above law of attraction, it follows that 
large bodies attract much more strongly than small ones, 
provided their densities are equal, and their distances the 
same ; and as the force of attraction constitutes the weight 
of a body, it follows that a body weighing a given num- 
ber of pounds on the earth, w r ould weigh much more on 
Jupiter or Saturn, and much less on Mercury or the 
Asteroids. 




65. Define gravitation. (W T hat illustration given ?) 

66. W^hat relation has gravitation to iveight? Upon what does it depend 
for its degree of force ? (Why is a cubic foot of cork lighter than the same 
bulk of lead?) 

67. W 7 hat effect have the bulk and density of the planets upon the weight 
of bodies on their surfaces? (State comparative weights. Illustration? 
Why not attractive force or weight in exact proportion to bulk ? How must- 
bodies be weighed to ascertain difference, and why ?) 



GRAVITATION. 43 



1. The following table shows the relative attractive force of the sun and planets 
body weighing one pound on the earth, would weigh, 

lb. oz. ! lb. oz. 



On Mercury 1 1^ 

tt Venus 15 

" Mars 8 

" Jupiter 2 8 



On Saturn 1 5| 

" Uranus 12£ 

" Neptune unknown" 

" The Sun 28 5$ 



2. A person weighing 150 lbs. on the earth would consequently weigh but 74 lbs. upon 
Mars; while upon Jupiter, his weight would be 375 lbs. ; and upon the sun, 4,250 lbs. I 
The attractive force of the Asteroids is so slight, that, if a man of ordinary muscular 
strength were transported to one of them, he might probably lift a hogshead of lead 
from its surface without difficulty. 

Z. But the learner will notice that the attractive force, as shown in the above table, is 
not in strict proportion to the bulk of the planets respectively. This difference will be 
accounted for by considering the difference in their density (63). From the principles 
there laid down," it will be seen at once, that though one planet be as large again as another, 
still, if it were but half as dense, it would contain no more matter than the smaller one, 
and their attractive force would be equal. If Jupiter, for instance, were as dense as the 
earth, his attractive force would be four times what it now is ; and if the density of all 
the solar bodies were precisely the same, their attractive force, or the weight of bodies on 
their surfaces, would be in exact proportion to their bulk. 

4. It must be remembered, however, that if a body were actually weighed upon the 
surface of each planet, by scales, it would weigh the same on all, because the force of 
attraction upon the weights would be just equal to that of the body to be weighed, 
whether it were more or less. With a steelyard it would be the same. A spring and 
hook, therefore, is the only instrument with which we could weigh objects accurately on 
the different planets. 

68. If the earth were only one-half as dense as she now 
is, it would reduce the weight of bodies at her surface 
one-half. So if a body were taken from the earth's sur- 
face half way down to her center, the weight would be 
reduced one-half. At her center it would be nothing, 
because the attractive force would be the same in all 
directions. 

In this cut, the diameter of the earth is divided into four 
equal parts — C, D, E, and F. At A, the whole attraction 
amounts to four pounds. When the stone reaches B, the 
part C attracts as strongly upward as D does downward, and 
their forces balance each other. Then as C and D mutually 
neutralize each other, we have only the parts E and F, or one- 
half the globe, to attract the stone downward ; consequently 
the attractive force would be only half as great at B as at A, 
and the stone would weigh only two pounds. 

69. The force with which bodies gravi- 
tate toward each other is in direct pro- 
portion to their respective masses, and in inverse propor- 
tion to the squares of their distances. 

A man carried upward in a balloon weighs less and less as his distance from the earth 
is increased. The same law holds good in regard to the planetary worlds. The nearer 
a planet is t» the sun, or to any other body, the stronger the mutual gravitation. 

68. How effect weight of bodies on the earth to reduce her density one- 
half? How to take down halfway to center ? Quite to center ? (Illustrate 
by diagram.) 

69. Give the exact law of gravitation ? (What said of a man ascending in 
a balloon ? Of more distant planets ?) 




u 



ASTRONOMY. 



70. This great law was discovered by Sir Isaac New- 
ton, in 1666. He was then only twenty-four years of 
age. 

The inquiry which led to the discovery is said to have been suggested to the mind of 
this youthful philosopher by seeing an apple fall from the limb of" a tree. " What drew 
these two globes $ie apple and the earth) together ?" 



PERIODIC REVOLUTIONS OF THE PLANETS. 

71. The planets all revolve aronnd the sun from west 
to east, or toward that part of the heavens in which the 
sun appears to rise. 

To assist his conception of the direction in which the planets revolve, the student 
may suppose that if the earth was in her orbit beyond the sun, at 12 o'clock, she would 
go what we should call eastward, which would be the same direction that we should 
call westward on the earth, at the same time ; as bodies revolving in a circle move in 
opposite directions on opposite sides of the circle. 

72. The passage of a planet from any particular point 
in its orbit, around to the same point again, is called its 
periodic revolution / and the time occupied in making 
such revolution is called its period, or periodic time. 
The periodic times of the principal planets are as fol- 
lows : 



Mercury 
Venus 
Earth . 
Mars . 



ears 


. Days. 





88 





225 


l 





l 


322 



Jupiter 
Saturn 
Uranus 
Neptune 



Years. Days. 

11 317 

29 175 

84 27 

164 226 



The Asteroids revolve, on an average, in about 4-£ years. 



1. The exact periods of the Asteroids, respectively, are as follows : 

Years. Days 

98 
207 
230 
250 
253 
284 
306 

45 



Flora 3 

Clio 3 

Vesta 3 

Iris 3 

Metis 3 

Hebe 3 

Parthenope 3 

Egeria 4 

2. The periodic time of a planet may very properly be called its year; hence a year 
of Saturn is equal to about thirty of ours, &c. But this difference in the length of the 
years of the several planets is not owing solely to the difference in the extent of their 
orbits. There is an actual difference in their velocities, as will be shown in the next 
paragraph. 



Years. 

Astrsea 4 

Irene 4 

Eunomia 4 

Juno 4 

Ceres . 4 

Pallas 4 

Hygeia 5 



Da vs. 

51 

55 
114 
134 

220 
225 
21T 



70. When and by whom were the Laws of Gravitation discovered ? How 
old ? (What led to this discovery ?) 

71. In what direction do the planets revolve in their orbits ? (Give illus- 
tration.) 

72. What meant by the periodic revolution of a planet ? Its period or peri- 
odic time f Give the periods of the principal planets. Of the Asteroids ? 
(What constitutes the year of a planet ? Compare years.) 



CENTRIPETAL AND CENTRIFUGAL FORCES. 45 



HOURLY MOTION OF THE PLANETS IN THEIR ORBITS. 

73. The velocity with which the planets fly through 
space, in performing their periodical journeys around the 
sun, varies from 11,000 to 110,000 miles an hour. The 
hourly motion of the earth amounts to 68,000 miles ! 

1. The hourly motion of the planets is, approximately, as follows: 

Miles. 

Mercury 110,000 

Venus 75,000 



Earth 6S,000 

Mars 55,000 



Miles 

Jupiter 30,000 

Saturn 22.000 

Uranus 15,000 

Neptune 11,000 



Here, instead of finding the swiftest planets performing the longest periodic journeys, 
this order is reversed, and they are found revolving in the smallest orbits. The nearer a 
planet is to the sun, the more rapid its motion, and the shorter its periodic time. The 
reasons for this difference in the velocities and periodic times of the planets, will appear 
in a subsequent paragraph. 

2. It may seem incredible to the student that the ponderous globe is flying through 
space at the rate of 6S,000 miles an hour, or some 80 times as" swift as a bullet; but, 
like many other astonishing facts in Astronomy, its truth can easily be demonstrated. 
The diameter of a circle is to its circumference as 7 is to 22 nearly. The earth's dis- 
tance from the sun beins: 95,000,000 miles, it is obvious that the whole diameter of her 
orbit is twice that distance, or 190,000,000; then, as 7 : 22 :: 190,000,000 : 597,142,857 
miles, the circumference of the earth's orbit. Divide this sum by 8,766, the number of 
hours in a year, and we have 6S,10S miles as the hourly velocity of the earth. 

3. As the earth is not propelled by machinery like a steamboat, or borne upon wheels 
like a railroad car, it is not strange that we are insensible of its rapid motion, especially 
as every thing upon its surface, and the atmosphere by which it is surrounded, move 
onward with it in its rapid flight. 

CENTRIPETAL AND CENTRIFUGAL FORCES. 

74. The mutual attractive force of the sun and planets 
is called the centripetal force ; while the tendency of the 
planets to fly off from the sun, as they revolve around 
him, is called the centrifugal force. 

1. The term centripetal is from centrum, center, and peto, to move toward ; and 
centrifugal, from centrum, and forgio, to fly from the center. 

2. The centrifugal force is generated by the revolution of the planet, and is in pro- 
portion to its velocity — the more rapid the revolution, the stronger the tendency to fly 
off from the sun. » 

3. If the centrifugal force were suspended, the planets would at once fall to the sun; 
and if the centripetal force were destroyed, the planets would fly off in straight lines, 
and leave the solar system forever. Then might be realized the chaos and confusion of 
the poet : 

"Let Earth unbalanced from her orbit fly, 
Planets and suns run lawless through the sky ; 
Let ruling angels from their spheres be hurled, 
Being on being wreck'd, and world on world." 

75. It has already been stated (65), that the force of 
attraction depends somewhat upon the distances of the 
attracting bodies — those nearest together being mutually 

73. What said of the velocity of the planets ? Of the earth ? (Table ? 
Remarks upon it? How is the hourly velocity of the earth ascertained? 
Why not sensible of this rapid motion ?) 

74. Centripetal and centrifugal forces ? (Derivation of terms ? How cen- 
trifugal force generated ? Suppose either suspended ?) 



46 ASTRONOMY. 



attracted most. It follows, therefore, that Mercury has 
the strongest tendency toward the sun, Venus next, the 
Earth next, &c, till we get through to Neptune ; and as 
the centrifugal force w x hich is to balance the centripetal is 
created by the velocity or projectile force of the planets, 
that velocity must needs be in proportion to their dis- 
tances, respectively, from the sun ; the nearest revolving 
the most rapidly. This we find to be the actual state of 
things in the solar system. 

The mechanism of the solar system strikingly displays the wisdom of the great 
Creator. The centrifugal force depends, of course, upon the rapidity of the revolution ; 
and in order that these forces might be exactly balanced, God has imparted to each 
planet a velocity just sufficient to produce a centrifugal force equal to that of its gravita- 
tion. Thus they' neither fall to the sun on the one hand, nor fly off beyond the reach of 
his beams on the other, but remain balanced in their orbits between these two great 
forces, and steadily revolving from age to age. " How manifold are thy works ! in wis- 
dom hast thou made them all." 

LAWS OF PLANETARY MOTION. 

76. Three very important laws, or principles, governing 
the movements of the planets, were discovered by Kep- 
ler, a German astronomer, in 1609. In honor of their 
discoverer, they are called Kepler's Laws. 

Kepler was a disciple of Tyclio Bra7ie, a noted astronomer of Denmark, and was 
equally celebrated with his renowned tutor. His residence and observatory were at 
"Wirtemburgh, in Germany. 

77. The first of these laws is, that the aphelion. 
orbits of all the planets are elliptical, 

having the sun in the common focus. 

The point in a planet's orbit nearest / \ 

the sun is called the perihelion point, j <<rilll> j 

and the point most remote the aphelion \ %^ 

point. Perihelion is from peri, about or \ 

near, and helios, the sun ; and aphelion, X., _,..--'' 

from apo, from, and helios, the sun. perihelion. 

From this first law of Kepler, it results that the plan- 
ets move with different velocities, in different parts of 
their orbits. From the aphelion to the perihelion points, 
the centripetal force combines with the centrifugal to 
accelerate the planet's motion ; while from perihelion to 

75. Why the planets nearest the sun revolve most rapidly in their orbits ? 
(Remark ?) 

76. Laws of planetary motion ? (Who was Kepler t) 

77. State the Jlrst of 'Kepler's laws, perihelion? Aphelion? 



LAWS OF PLANETARY MOTION. 



47 



aphelion points, the centripetal 
acts against the centrifugal force, 
and retards it. 

1. From A to B in the diagram, the centrifugal force, 
represented by the line C, acts icith the tendency to 
revolve, and the planet's motion is accelerated ; but 
from B to A the* same force, shown by the line D, 
acts against the tendency to advance, and the planet 
is retarded. Hence it comes to Aphelion with its 
least velocity, and to Perihelion with its greatest. 

2. In the statement of velocities on page 45, the 
mean or average velocity is given. 



it 

\ 



\0 



r>.< 






Jl 



RADIUS VECTOR. 
0- 



78. The second law is, that the \ / 
radius vector of a planet describes *\ b y'' 
equal areas in equal times. The 

radius is an imaginary line joining the center of the 

sun and the center of the planet, in any part of its orbit. 

Vector is from veho, to carry ; 

hence the radius vector is a radius 

carried round. By the statement 

thaU'tf describes equal areas in equal 

times, is meant that it sweeps over 

the same surface in an hour, when 

a planet is near the sun, and moves 

swiftly, as, when furthest from the 

sun, it moves most slowly. 

The nearer a planet is to the sun, the more rapid 
its motion. It follows, therefore, that if the orbit of a 
planet is an ellipse, with the sun in one of the foci, its 
rate of motion will be unequal in different parts of 
its orbit — swiftest at perihelion, and slowest at aphe- 
lion. From perihelion to aphelion the centripetal more directly counteracts the cen- 
trifugal force, and the planet is retarded. On the other hand, from the aphelion to the 
perihelion point, the centripetal and centrifugal forces are united, or act in a similar 
direction. They consequently hasten the planet onward, and its rate of motion is con- 
stantly accelerated. Now suppose, when the planet is at a certain point near its peri- 
helion, we draw a line from its center to the center of the sun. This line is the radius 
vector. At the end of one day, for instance, after the planet has advanced considerably 
in its orbit, Ave draw another line in the same manner to the sun's center, and estimate 
the area between the two lines. At another time, when the planet is near its aphelion, 
we note the space over which the radius vector travels in one day, and estimate its area. 
On comparison, it will be found, that notwithstanding the unequal velocity of the planet, 
and consequently of the radius vector, at the two "ends of the ellipse, the area over 
which the radius vector has traveled is the same in both cases. The same principle ob- 
tains in every part of the planetary orbits, whatever may be their ellipticity or the mean 
distance of the planet from the sun ; hence the rule, that the radius rector describes 
equal areas in equal times. In the preceding cut, the twelve triangles, numbered J, 2, 
3, &c, over each of which the radius vector sweeps in equal times, are equal. 

79. The third law of Kepler is, that the squares of the 




78. State the second law of planetary motion, 
plain this second law.) 



Define radius vector. (Ex- 



4S 



ASTRONOMY. 



MAR3 IN euNJi'NCTION 

e 

6 



6 



periodic times qf any two 2>lanett< are proportioned to the 
cubes of their mean aistances from the sun. 

1. Tike, for example, the earth and Mars, whose periods are 366*2564 and 686*9796 
days, and whose distances from the sun arc in the proportion <>i l to 1*52869, and it will 
be found that (865*2564)«: (686*9796)* ::(1) 8 : (l'M809)». 

'2. According to these laws, which arc known to prevail throughout the solar system, 

many of the facts of astronomy are deduced from other tacts previously ascertained, 
They are. therefore, of neat importanoe, and should be studied till they are, at least, 

thoroughly understood, if not committed to memory. 

ASPECTS OF THE PLANETS. 

80. By the aspects of the planets is meant their posi- 
tions in their orbits with respect to each other. The 
principal aspects 
are conjunction^ 
quadrature, nnd op- 
position. Two bod- 
ies are in conjunc- 
tion when in the 
same longitude ; 
that is, on the same 
north and south 
line in the heavens. 
The sign for con- 
junctionis 6. When 
90° apart, bodies 
are said to be in 
quadrature* with 

the Bign D ; and 

when 180° apart, ov 

in opposite parts of 

the heavens, they are in opposition, and the Bign is 8. 

Conjunctions are of two kinds. An inferior conjunction. 

is when the planet is between the earth and the sun; 

and a superior conjunction, when it is beyond the sun. 

1. Let the student Imagine himself stationed upon the earth in the oni Then the 
sun and three planets above :nv in conjunction* The inferior and superior are distin- 
guished ; while at A, i planet is shown in quadrature and at the bottom of the eut the 

planet Mars in opposition With the sun and interior planet. 



©A 




V 



6 , 



MARS 



<? ... -' 

N OPPOSITION 



79. state the third law, (Illustration ! What said of the importanoe of a 

knowledge Of these laws . ; > 

80. What meant by the aspect* of the planets? state principal aspect* 1 
Define each, Signal How many kinds of conjunctions ? Define each. 
(Explain by diagram. When is Venue nearest I what difference at superior 

uiul interior conjunctions I) 



SIDEREAL AND SYNODIC REVOLUTIONS. 49 



2. When at her superior conjunction, Venus is 154 millions of miles from (ho earth ; 
but when at her inferior conjunction, she is only 26 millions of miles distant, or the whole 
diameter of her orbit nearer. 



SIDEREAL AND SYNODIC REVOLUTIONS. 

81. The sidereal revolution of a planet is a complete 
revolution from any given point in its orbit around to the 
same point again. 

Sidereal, from tiderdUi — a revolution as measured by the stars. See page 98, note 1. 
The periodic revolutions of the planets, given at Art 71, are sidereal revolutions. 

82. A synodic revolution is from one conjunction to 
the same conjunction again. 

1. The term synod signifies a meeting or convention ; and the synodic revolution of 
a planet is a wetting revolution: that is, from one meeting or conjunction to another. 

2, The difference between B sidereal and synodic revolution may he illustrated hy the 
motion of the hands of a clock or watch. At twelve, o'clock, the hour and minute hands 
■re together; but at one o'clock, when the minute-hand has made a complete revolu- 
tion, and points to XII. again, the hour-hand has gone forward to I., and the minute- 
hand will not overtake it till about li\e minutes afterward. The revolution of the hour- 
hand from XII. to XII. again represents the sidereal revolution of a planet ; and when 
it overtakes the hour-hand, it becomes a synodic revolution. 

8. The sidereal and synodic periods of the principal planets are as follows: 

Sidereal. Synodic. 

Mercury — 88 days 115 days. 

Venus — 225 "* 504 " 

Mars 1 year, 822 " 780 " 

Jupiter 11*" 317 " 899 " 

Saturn 20 " 175 " 3T8 " 

Uranus 84 " — 869| " 

Neptune 10 1 " 22G " 867J " 

From this table it is seen that the synodic periods of the moro distant planets corre- 
spond very nearly with the periodic time of the earth. Being remote from the sun, they 
move very slowly, and the earth soon overtakes them, after performing her periodic 
revolution. 

BTOODIO PXEIOD8 OF Tin: EXTERIOR PLANETS. 



SZP 



V. 

I)— 

V"" 






Suppose the earth and Uranus to be in conjunction, as shown at A \l. In 8664 days, 
the earth performs her sidereal or periodic revolution, and returns to the point 
A again. In the mean time Uranus, whoso periodic time is 84 years, has passed 

81. What meant by the sidereal revolution of a planet? (Derivation of 
term? Arc the periods of the planets sidereal revolutions f) 

82. St/twdic revolution? (Illustrate diilerenee by clock. What fact re- 
specting synodic periods of distant planets ? How explained ? Illustrate by 
diagram.) 



50 ASTRONOMY. 



through only -g^th part of his orbit, or about 4^° to the point C; and in 4£ days the 
earth overtakes him on the line D. It is on this account that the synodic period of 
Uranus is only 367£ days, or 4£ days longer than the periodic time of the earth. 



THE ECLIPTIC, ZODIAC, SIGNS, ETC. 

83. The Ecliptic is the plane of the earth? s orbit, or 
the path in which the sun appears to revolve in the 
heavens. 

PLANE OP THE ECLIPTIC. 




1. In the above cut, an attempt is made to represent the ecliptic, or plane of the 
earth's orbit. It is an oblique view, which makes the orbit appear elliptical. It shows 
one-half of the sun and half the earth on one side, and half on the other. The circle 
projecting beyond the orbit is to represent the plane of the ecliptic, indefinitely ex- 
tended. 

2. If the student has any difficulty in getting a correct idea respecting the ecliptic, 
let him suppose the orbit of the earth to be a hoop of small wire laid upon a table : the 
surface of the table, both within and without the hoop, would then represent the plane 
of the ecliptic. From the above definition and description, it will be seen that the eclip- 
tic passes through the center of the earth, and the center of the sun ; consequently the 
ecliptic and the apparent path of the sun through the heavens are in the same plane. 
It will be easy, therefore, to ascertain the true position of the ecliptic in the heavens, and 
to imagine its course among the stars. 

3. The plane of the earth's orbit is called the ecliptic, because eclipses of the sun 
and moon never take place except when the moon is in or near this plane. 

84. The position of the ecliptic to persons north of the 
equator is south of us. It runs east and west, cutting 
the centers of the sun and earth. North of the ecliptic 
is called above it; and south of it, below it. 

The student should again be reminded that there is no absolute up or down in the 
universe. He must also guard against the idea that the ecliptic may be horizontal. This 
term has reference only to the earth, and is descriptive of a plane depending altogether 
for its own position upon that of the observer, as shown and illustrated at 20. Though 
the ecliptic is a permanent plane, and cuts the starry heavens around us at the same 
points from age to age, it has no absolute up or dotcn, unless it should be the direction 
to and from the sun. The distinction of above and below is merely arbitrary, and grows 
out of our position north of the equator, which makes the south side of the ecliptic ap- 
pear down to us. 



83. What the ecliptic? (How cut the earth and sun ? Point out its course 
in the heavens. Why called the ecliptic ?) 

84. What meant by above and below the ecliptic ? (Remarks in note.) 



ECLIPTIC, ZODIAC, SIGNS, ETC. 51 



85. The Poles of the Ecliptic are the extremities of 
an imaginary axis upon which the ecliptic seems to 
revolve. 

As the ecliptic and equinoctial are not in the same plane, their poles do not coincide, 
or are not in the same points in the heavens. The cause of this variation will be ex- 
plained hereafter. 

86. The Zodiac is an imaginary belt 16° wide, viz., 
8° on each side of the ecliptic, and extending from west 
to east quite around the heavens. In the heavens, it in- 
cludes the sun's apparent path, and a space of eight de- 
grees south, and eight degrees north of it. 

THE ECLIPTIC AND ZODIAC. 




••■0--..^/.:.^ #-' 




In this cut, the interior dotted circle represents the earth's orbit; the exterior the 
plane of her orbit extended to the starry heavens. The dark lines each side of the 
ecliptic are the limits of the zodiac. The earth is shown in perspective, largest near to 
us, and growing smaller as her distance is increased. The arrows show her direction. 

87. The great circle of the zodiac is divided into twelve 
equal parts, called signs. (These divisions are shown in the 
above cut, by the spaces between the perpendicular lines 
that cross the zodiac.) The ancients imagined the stars 
of each sign to represent some animal or object, and gave 
them names accordingly. On this account, they gave 
the name zodiac to this belt around the heavens ; not, as 
some have imagined, because it was a zone, but from the 
Greek zoon, an animal, because so many animals were 
represented within its limits. 



85. The poles of the ecliptic ? (Do the poles of the ecliptic and the poles 
of heavens coincide ?) 

86. What is the zodiac ? 

87. How is the zodiac divided ? Idea of the ancients ? Origin of the 
name zodiac ? 



52 



ASTEONOMY. 



88. The names, order, and symbols of the twelve signs 
of the zodiac are as follows : 



Aries (or the Earn) . . . . T 
Taurus (the Bull) ..... X 
Gemini (the Twins .... II 

Cancer (the Crab) S> 

Leo (the Lion) £1 

Virgo (the Virgin) . . . . W 



Libra (the Balance) — 

Scorpio (the Scorpion) . fll 
Sagittarius (the Archer) / 
Capri cornus (the Goat) V3 
Aquarius (theWaterm an) ^ 
Pisces (the Fishes) . . . . X 



These names being from the Latin, their signification is added in parentheses, and should 
be understood by the pupil. In reciting, however, it is only necessary to give the first 
names — as Aries, Taurus, Gemini, &c. By carefully observing these symbols, the stu- 
dent will detect a resemblance between several of them and the objects they represent. 
For instance, the sign for Aries represents his horns ; so also with Taurus, &c. 



ANCIENT ASTROLOGY. 



89. The ancients pretended to predict future events by 
the signs, aspects, &c. This art, as it was called-, was 
denominated Astrology. Astrology was either natural 
or judicial. Natural Astrology aimed at predicting re- 
markable occurrences in the natural world, as earthquakes, 
volcanoes, tempests, and pestilential diseases. Judicial 
Astrology aimed at foretelling 
the fates of individuals or of em- 
pires. 

" This science," says Webster, " was formerly in 
great request, as men ignorantly supposed the heaven- 
ly bodies to have a ruling influence over the physical 
and moral world ; but it is now universally exploded 
by true science and philosophy." A fragment of this 
ancient superstition, like the adjoining figure, may 
still be met with occasionally in the pages of an al- 
manac ; and there are still persons to be found in al- 
most every community who think certain " signs" 
govern certain portions of the human body, and that 
it is very important to do every thing " when the 
sign is right." Impostors, also, are still taking advan- 
tage of this credulity; and, professing to "tell for- 
tunes," as they call it, by the stars, impose upon and 
defraud the ignorant. The stars have no more to do 
with our " destiny" th^m we have with theirs. 

90. The.order of the signs is from west to east around 
the heavens. Thus Aries, Taurus, Gemini, &c, around 
to Pisces. 




88. Names of the signs ? Symbols on blackboard. 

89. What is astrology ? How divided ? Define each. (Remark of Web- 
ster ? Of the author ?) 

90. The order of the signs ? (Describe the cut. What said of Taurus ?) 



CELESTIAL LATITUDE AKD LONGITUDE. 

PERPENDICULAR VIEW OF THE ECLIPTIC. 

W> 9 350 



53 




091 ozi 'oTW * 

On pages 50 and 51, we presented oblique views of the ecliptic. The above is a p& ■ 
pendicular view. The sun is seen in the center, and the earth revolving around him ; 
and in the distance is shown the circle of the starry heavens. This circle is divided 
into twelve equal parts, representing the twelve signs ; while the object which the stars 
in each sign were supposed to resemble is placed in that sign, and the symbol imme- 
diately opposite and within the sign. But the head of Taurus should point east instead 
of west. 

CELESTIAL LATITUDE AND LONGITUDE. 

91. Celestial Longitude is distance east of a given 
point in the heavens, reckoned on the ecliptic. Begin- 
ning at the Vernal Equinox, it is reckoned eastward to 
360°, or to the point whence we started. 

The pupil will consult the preceding cut in which the longitude is marked for every 
ten degrees. By holding the book up to the south of him, the surface of the page will 
represent the plane of the ecliptic ; and the reckoning of 10, 20, 30, &c, from the top of 
the cut eastward, will answer to the manner in which celestial longitude is reckoned 
eastward around the heavens. 

92. Celestial Longitude is either Heliocentric or Geo- 
centric. The heliocentric longitude of a planet is its 
longitude as viewed from the sun ; and the geocentric, its 
longitude as viewed from the earth. 

Geocentric is from ge, the earth, and keniran, center ; and heliocentric from helios, 
the sun, and kentron, center. 



91. Celestial longitude? Where begin to reckon? Illustrate by book. 
Point out order of reckoning in the heavens. 

92. What is heliocentric longitude ? Geocentric? (Derivation of terms ? 
Illustrate by diagram.) 



54 



ASTRONOMY. 



GEOCENTRIC AND HELIOCENTRIC LONGITUDE. 




In this cut, the planet B, when viewed from the earth at A, seems to be in the sign 
Co ; but when viewed from the sun, it appears to be in n. Again : when at C, her 
apparent longitude from the earth is in rrt ; when from the sun, she appears to be in t . 
The reader will not only perceive the difference between geocentric and heliocentric 
longitude, but will see why the latter more than the former indicates the true position 
of the planet. It is an easy thing, however, if one is known, to deduce the other from it. 



MEAN AND TRUE PLACES 
OF A PLANET. 

93. The mean place 
of a planet is the place 
it would have occupied 
had it revolved in a cir- 
cular orbit, and with 
uniform velocity. 

The true place is that 
which it really occupies, 
revolving as it does in 
an elliptical orbit, and 
with unequal velocity. 

1. In the cut, the dotted ellipse 
represents the orbit of the planet, 
and the points T T T, &c, its true 
place. In the circle or hypothetical 
orbit, the points M M, &c, indicate 
the mean place of the planet. 



MEAN AND TRUE PLACES OF A TLANET. 




93. What is meant by the mean place of a planet ? The truepUce ? (When 



DIRECT AKD RETROGRADE MOTIONS. 



55 



2. From the perihelion to the aphelion, it will be seen that the true place is in ad- 
vance, or eastward, of the mean place ; while from aphelion to perihelion again, the 
mean place is in advance of the true. But at the perihelion and aphelion points, the 
mean and true places coincide. 

3. In one respect, the cut conveys an erroneous impression, as it represents the planet 
as passing over an equal distance in its orbit in equal times. This is not the fact. The 
difference in its velocity in different parts of its orbit could not well be represented here ; 
but the student will find it beautifully illustrated by the second cut on page 47, and in 
the explanatory note accompanying it. 

94. Celestial Latitude is distance north or south of 
the ecliptic, and is reckoned to the pole of the ecliptic, 
or to 90°. 



DIRECT AND RETROGRADE MOTIONS. 




DIRECT AND RETROGRADE MOTIONS. 

95. The apparent motion of a planet is said to be 
direct when it is eastioard among the stars, and retrograde 
when it seems to go back or westward in the ecliptic. 
When it seems to move neither east nor west, it is said to 
be stationary. 

96. The cause of the appa- 
rent retrogression of the in- 
terior planets is the fact 
that they revolve much more 
rapidly than the earth, from 
which we view them ; causing 
their direct motion to appear 
to be retrograde. 

Suppose the earth to be at A, and Venus at 
B, she would appear to be at C, among the 
stars. If the earth remained at A while Ve- 
nus was passing from B to D, she would 
seem to retrograde from C to E ; but as the 
earth passes from A to F while Venus goes 
from B to D, Venus will appear to be at G ; 
and the amount of her apparent westward \ 
motion will only be from C to G-. 

97. The apparent retro- 
grade motions of the exterior 
planets is due to the rapidity 

with which the position from which we view them is 

is the true in advance of the mean? When the reverse? When do they 
coincide ? Wherein is the cut defective ? Where have we a true represen- 
tation ?) 

94. Celestial latitude ? How reckoned ? 

95. When is a planet's apparent motion direct? Retrograde? When is 
a planet said to be stationary ? 

96. State the cause of the apparent retrogression of an interior planet. 
(Illustrate by diagram.) 

97. The cause of retrogression of exterior planets. (Illustrate by diagram. 
What fact shown ?) 




% 



56 ASTRONOMY, 



changed, as we are carried rapidly through space with the 
earth, in her annual journey around the sun. 



RETROGRADE MOTION 07 THR EXTERIOR PLANETS. 



If 



A 



— T i* %& h 

1. Suppose the earth at A, and the planet Neptune at B, he would then appear to 
be at C, among the stars ; but as Neptune moves but a little from B toward F, while the 
earth is passing from A to D, Neptune will appear to retrograde from C to E. What- 
ever Neptune may have moved, however, from B toward F, will go to reduce the 
amount of apparent retrogression. 

2. It is obvious from this figure, that the more distant an exterior planet is, and the 
slower it moves, the less will be its arc of retrogradation, and fhe longer will it be retro- 
grading. Neptune appears to retrograde 180 days, or nearly half the year. 

98. The portion of the ecliptic through which a planet 
seems to retrograde is called the Arc of. Retrogradation. 
The more remote the planet the less the arc, and the 
longer the time of its retrogression. 

The following table exhibits the amount of arc and the time of the retrogradation of 
the principal planets : 

Arc. Days. 

Mercury 13|° 23 

Venus 16 42 

Mars 16 73 

Jupiter 10 121 

Saturn 6 139 

Uranus 4 151 

Neptune 1 180 

99. The greatest elongation of an interior planet is the 
greatest apparent distance east or west of the sun at 
which it is ever found. 

In the second cut back, the point B would represent the greatest eastern, and D the 
greatest western, elongation of the planet. At these two points she would appear to be 
stationary. 

100. The greatest elongations of Venus vary from 45° 
to 48°. The fact that she never departs more than 48° 
from the sun proves that her orbit is within that of the 
earth ; and the variation in her elongations shows that 
her orbit is not an exact circle. 

98. What meant by the Arc of Retrogradation? 

99. Greatest elongation ? 

100. Greatest elongation of Venus ? What does it prove ? 



VENUS AS MORNING AND EVENING STAR. 



57 



101. When Venus is west of the sun, and rises before 
him, she is morning star / and when east of the sun, she 
is evening star. 



VENUS AS MORNING AND EVENING STAR. 
Al 







1. Let the student hold, the book up south of him, and he will at once see why Venus 
is alternately morning and evening star. Let the plane A B represent the sensible or 
visible horizon, C D the apparent daily path of the sun through the heavens, and E 
the earth in her apparent position. The sun is shown at three different points — 
namely, rising in the east, on the meridian, and setting in the west; while Venus is 
seen revolving around him from west to east, or in the direction of the arrows. Now 
it is obvious that when Venus is at F, or west of the sun, she sets before him as at G-, 
and rises before him as at II. She must, therefore, be morning star. On the other 
hand, when she is east of the sun, as at J, she lingers in the west after the sun has gone 
down, as at K, and is consequently evening star. 

2. In this cut, Venus would be at her greatest elongation eastward at J, and west' 
ward at F, and in both cases would be M stationary.'''' At L and M she would be in 
conjunction with the sun. 

3. Were the earth to suspend her daily rotation, with the sun on the meridian of the 
observer, as represented at L, we might readily watch Venus through her whole circuit 
around the sun. 

4. Venus may sometimes be seen at mid-day, either east or west of the sun, and Dr. 
Dick considers the day-time most favorable for observing her with a telescope. 

102. Venus is morning and evening star, alternately, 
for about 292 days, or from one conjunction to another. 
Appearing first east and then west of the sun, she was 
regarded by the ancients as two different stars, which they 
called Phosphor and Hesperus. 

When Venus is near her greatest elongation from the sun, she is one of the most beau- 
tiful stars in the heavens. She is very easily found, either just before sunrise, or just 
after sundown ; and we earnestly recommend the class to ascertain where she is, at the 
time of learning this lesson, and to watch her movements for a few months, and see if 
they do not correspond with the description here given. The knowledge acquired will 
thus be located in the heavens. 



101. When morning and when evening star ? 

102. How long is Venus alternately morning and evening star ? 
yarded by the ancients ? (Remark in note ?) 

3* 



How re- 



58 



ASTRONOMY. 



103. The greatest elongations of Mercury vary from 16 
to 29 degrees from the sun, which proves his orbit to be 
elliptical, and to be within that of Venus. 

1 As Mercury never departs more than 29° from the sun, when at his greatest elonga- 
tion, and Venus is never nearer than about 45°, when at her greatest elongation, it is 
evident that his orbit is inside that of Venus. 

2 When at perihelion, Mercury is only 29,305,000 miles from the sun s center ; while 
in the opposite part of his orbit, or in aphelion, he reaches to 44,474,000— making a vari- 
ation of distance, arising from the ellipticity of his orbit, of more than 15,109,000 miles, 
which is nearly five times as great as in the case of the earth. 

104. In consequence of the nearness of Mercury to the 
sun, he is very rarely seen ; and if seen at all, it must be 
in strong twilight, either morning or evening. He never 
appears conspicuous, even under the most favorable cir- 
cumstances, but twinkles like a star of the third magni- 
tude, with a pale rosy light. 

By consulting an almanac, the student can ascertain when Mercury is at his greatest 
elongation, and if it is eastward, look out for him low down in the west, just after sun- 
set. If his elongation is westward, he must be looked for in the east, before sunrise. 
It will be worth rising early to see him. 

DEVIATION OF THE ORBITS OF THE PLANETS FROM THE PLANE 

OF THE ECLIPTIC. 

105. Although the sun is the great center around 
which all the planets revolve, it should be borne in mind 
that no two of them revolve in the same plane. Taking 
the plane of the earth's orbit or ecliptic as the standard, 
the orbits of the other planets all depart from that plane, 
some more and some less. As a consequence, they all 
pass through or cut the plane of the ecliptic twice at 
every revolution. 

VKNIT8 PASSING AND REPASSING THE PLANE OF TIIE EARTH'S ORBIT. 

L 




In this cut, the space included within the orbit of the earth is tinted to represent a 
plane. Within her orbit, and part above, and part below it, may be seen the orbit of 

103. Greatest elongation of Mercury ? Proves what ? (Show how demon- 
strated. What said of the eccentricity of the orbit of Mercury ?) 

104. Is Mercury often seen ? When, if at all ? Appearance ? (How 
find?) , . W1 

105. Are all the planetary orbits in the same plane ? W hat consequence 
follows '! 



DEVIATION OF THE OKBITS OF THE PLANETS. 



59 



Venus, the arrows showing her direction. Her orbit goes out of sight when it passes 
the plane of the ecliptic. 

106. The points where a planet passes the plane of the 
ecliptic are called the Nodes of its orbit. They are in 
opposite sides of the ecliptic, and of course 180° apart. 
The point where they pass south of the ecliptic is called 
the descending node, and marked Z3 ; and that through 
which they pass north of the ecliptic is called the ascend- 
ing node, and marked &. The Line of the Nodes is a 
line drawn from one node to the other across the ecliptic. 

The nodes, ascending and descending, and their symbols, and also the line of the nodes, 
marked L N, are ail well represented in the preceding cut. 

INCLINATION OF THE ORKITS OF THE PLANETS TO THE PLANE OF THE ECLIPTIC. 




107. The nodes of the planetary orbits are not all in 
the same longitude, but are distributed all around the 
ecliptic. In astronomical works and calculations, the 
longitude of the ascending node only is noted, as the 
opposite node is always just 180° from it. 

The longitude of the ascending nodes of the planets, respectively, is as follows : 



Mercury 46° 

Venus 75 

Earth — 

Mars 4S 

Flora 110 

Clio — 

Vesta 103 

Iris 260 



Metis G90 

Hebe 13S 

Parthenope — 

Egeria — 

Astrsea 141 

Irene — 

Eunomia — 

Juno 171 



Ceres 81° 

Pallas 173 

Hygeia — 

Jupiter 98 

Saturn 112 

Uranus 72 

Neptune 130 



106. What are the Nodes of a planet's orbit ? Ho.w situated with respect to 
each other ? What called respectively, and why ? What meant by the line 
of the nodes ? 

107. Are the nodes of all the planetary orbits in the same longitude ? How 
distributed ? W 7 hich node usually mentioned and, located ? Why not both ? 



GO ASTRONOMY. 



10S. The deviation of the planets, respectively, from 
the ecliptic varies from 1° 46" to 34^°. The orbits of the 
larger planets are all near the ecliptic, while some of the 
asteroids depart widely from it. On this account they are 
sometimes called ultra-zodiacal planets. 

The preceding cut may help the student to form an idea of the inclination of the 
planetary orhitsi but we must guard against the impression it may make that all the 
planetary nodes are in the same pari o/t/ie ecliptic, as we were obliged to represent in 
the cut." Instead of this, they are distributed all about the ecliptic. Again: the cut 
shows the several planets at about the same distance from the sun, contrary to the fact, 
as stated and illustrated on page 80. The dotted line represents the earth's orbit, or 
plane of the ecliptic, and the other lines the planes of the orbits of several of the plan- 
ets, and their departure from the ecliptic. The inclination of the several orbits is, in 
round numbers, as follows: 



Metis <v"r>4' 

Hebe lb? 47 

Parthenope — 

Egeria — 

Flora 5° 58* A>tnva 5° 19' 

Olio T° OS' Irene — Uranus 1° 46 

Vesta 7° OS' Kunomia — Neptune 1° 46 

Iris 5^-2S" Juno 10^ 3'. 



Mercury ...7° 

Venus.*. BOW 

Earth — 

Mars IO SV 



Ceres 10^87' 

Pallas 84^37' 

Hysela — 

Jupiter 1°1S' 

Saturn >2° 29' 



OF TRANSITS. 

109. The passage of a heavenly body across the me- 
ridian of any place, or across the disk of the sun, is 
called a transit. A planet will seem to pass over the 
disk of the sun when it passes directly between us and 
him ; and as none but the interior planets can ever get 
between us and the sun, it is obvious that no others can 
ever make a transit over his disk. 

The term transit is sometimes used with reference to terrestrial objects, as when we 
speak of the transit or passage of goods through a country. The words transition^ 

transitire, transitory, &o, are derived from the primitive word transit. 

110. Mercury and Yeans are the only planets that can 
appear to cross over the sun's disk, as viewed from our 
globe. 

Were we stationed upon one of the remote exterior planets, we might see the earth, 
and Mars, and Jupiter transit the sun; but as it is, we shall never witness such phe- 
nomena, or, at least, till we leave the present world. 

111. Were the orbits of Mercury and Venus in the 
same plane with that of the earth, they would transit the 

108, To what extent Jo the planetary orbits depart from the ecliptic ? 
What said of the larger planets ? Of the smaller ? ^Kemarks upon the out. 
State the inclination of Mercury, Venus, Mars, &o.) 

109, What is a transit t When do planets transit the sun \ What planets 
do this I Why not the exterior : ^Kemarks upon term transit.) 

110, What planets make transits across the sun's disk i ^Remarks in 
note.^ 



TRANSITS. 



01 



sun at every synodic revolution ; but as one-half of each 
of their orbits is aboiw, and the other half helow the 
ecliptic, they generally appear to pass either above or 
below the sun. 



n 



B 

- •■' 

A. 





5© 



Let the right line A, joining the earth and the sun in the ahove diagram, represent 
the plane of the ecliptic. Now when an Interior planet is in this plane, as shown at A, 
it may appear to be upon the sun's disk; but if it is either above or below the ecliptic, 
as shown at 1> and 0, it will appear to pass cither above or below the sun, as shown at 
1) and E. 

112. A transit can never occur except when the inte- 
rior planet is in or very near the ecliptic. The earth and 
the planet must be on the same side of the ecliptic ; the 
planet being at one of its nodes, and the earth on the 
line of its nodes. 



PHILOSOPHY OF TRANSITS. 




This cut represents the ecliptic and zodiac, with the orbit of an interior planet, his 
nodes, &0. The line of his nodes is, as shown, in the 10° of » and the 16° of ill. 
Now if the earth is in a, on the line L N, as shown in the cut, when Mercury is at 
his ascending node {h\ he will seem to pass upward over the sun's face, like a dark 
spot, as represented in the figure. On the other hand, if Mercury is at his {J when 
the earth is in the 1G° of HI, the former will seem to pass doicnica'rd across the disk of 
the sun. 

113. As the nodes of the planetary orbits are in oppo- 

111. Why not transits every revolution of Mercury and Venus? (Illus- 
trate by diagram.) 

1VJ. When must transits occur, if at all? Where must the earth and 
planet be \ (Illustrate by diagram.) 



62 



ASTRONOMY. 



site sides of the ecliptic, it follows that the earth must 
pass the line of the nodes of the interior planets, re- 
spectively, in opposite months of the year. These 
months are called the node months of the planet, and are 
the months in which all its transits must occur. 

114. In making transits across the sun's disk, the 
planets seem to pass from east to west, and to ascend or 
descend, as respects the ecliptic, according as the planet 
is at the ascending or descending node. 

This variation in the direction of the planets, during different transits, is well repre- 
sented in the next cut. 

115. The node months of Mercury are May and No- 
vember. 

All the transits of Mercury ever noticed have occurred in one or the other of these 
months, and for the reason already assigned. The first ever observed took place 
November 6, 1681; since which time there have been 29 others by the same planet— 
in all 80— S in May, and 22 in November. 



The last tran- 

Mercury oc- 

November 9, 

and the next 



TRANSITS OF MERCURY. 

UQRTH 



116. 
sit of 
curred 
1818 ; 

will take place No- 
vember 11, 1861. Be- 
sides this, there will 
be five more during 
the present century — 
two in May, and three 
in November. 

The accompanying cut is a de- 
lineation of all the transits of 
Mercury from 1S02 to the close 
of the present century. The 
dark line running east and west 
across the sun's center represents SOUTH 

the plane of the ecliptic, and tho 
dotted lines the apparent paths 

of Mercury in the several transits. Tho planet is shown at its nearest point to the sun's 
center, ltis path in the last transit and in the next will easily be found. 

8. The last transit of Mercury was observed in this country by Professor Mitchel, at 
the Cincinnati Observatory, and by many others both in America and in Europe. 




113. What are the node months ? (Explain by diagram.) 

114. In what direction do planets cross the sun in transits, and why ? 

115. Which are the node months of Mercury ? 

116. When did the last transit of Mercury occur ? When will the next 
take place ? (What represented in the cut. ? 'Describe. Where is the planet 
shown ? What said of last transit of Mercury ?) 



TRANSITS. 63 



The writer had made all necessary preparation for observing the phenomenon at his 
residence, near Oswego, New York ; but, unfortunately, his sky was overhung with 
clouds, which hid the sun from his view, and disappointed all his hopes. 

117. The node months of Venus are December and 
June. The line of her nodes lies in Gemini (II) and 
Sagittarius (/); and as the earth always passes those 
points in the months named, it follows that all transits of 
Venus must occur in those months for ages to come. 

This proposition will be well understood by consulting the cut on page 61 ; for as the 
line of Venus's nodes is only one sign ahead of that of Mercury, the earth will reach 
that point in the ecliptic in one month after she passes the line of Mercury's nodes ; so 
that if his transits occur in May and November, hers should occur in June and Decem- 
ber, as is always the case. 

118. The last transit of Venus occurred December 6, 
1822 ; and the next will take place December 8, 1874. 

1. Only four transits of Venus have as yet been observed — namely, December 4, 1G39 ; 
June 5, 17G1 ; June 3, 1769 ; and December 6, 1822. It is said that Rittenhouse was so 
interested in viewing that of 1769, that he actually fainted. In defining the term tran- 
sit. Dr, Webster says: "I witnessed the transit of Venus over the sun's disk, June 3, 
1769.' 1 See "Unabridged" Dictionary. The next four will occur December 8, 1874 ; 
December 5, 18S2 ; June 7, 2004 ; and June 5, 2012. 

2. The first transit ever witnessed was that of December 4, 1639. The observer was 
a young man named Horrox, living in an obscure village near Liverpool, England. The 
table of Kepler, constructed upon the observations of Tycho Brahe, indicated a transit of 
Venus in 1631, but none was observed. Horrox, without much assistance from books 
and instruments, set himself to inquire into the error of the tables, and found that such 
a phenomenon might be expected to happen in 1639. lie repeated his calculations 
during this interval with all the carefulness and enthusiasm of a scholar ambitious of 
being the first to predict and observe a celestial phenomenon which, from the creation of 
the world, had never been witnessed. Confident of the result, he communicated his ex- 
pected triumph to a confidential friend residing in Manchester, and desired him to watch 
for the event, and to take observations. So anxious was Horrox not to fail of witnessing 
it himself, that he commenced his observations the day before it was expected, and re- 
sumed them at the rising of the sun on the morrow. But the very hour when his cal- 
culations led him to expect the visible appearance, of Venus on the sun's disk, was also 
the appointed hour for tlie public worship of God on the Sabbath. The delay of a few 
minutes might deprive him forever of an opportunity of observing the transit. If its 
very commencement were not noticed, clouds might intervene, and conceal it until the 
sun should set ; and nearly a century and a half would elapse before another opportunity 
would occur. He had been waiting for the event with the most ardent anticipation for 
eight years, and the result promised much benefit to the science. Notwithstanding all 
this, Horrox twice suspended his observations, and twice repaired to the house of God, 
the great Author of the bright works he delighted to contemplate. When his duty was 
thus performed, and he had returned to his chamber the second time, his love of science 
was gratified with full success, and he saw what no mortal eye had observed before. If 
any thing can add interest to this incident, it is the modesty with which the young 
astronomer apologizes to the world for suspe?iding his observations at all. " I observed, 
it," says he, "from sunrise till nine o'clock, again a little before ten, and lastly at noon, 
and from one to two o'clock ; the rest of the day being devoted to higher duties, which 
might not be neglected for these pastimes." 

3. The transit of 1769 was observed with intense interest by astronomers in both hemi- 
spheres. To secure the advantages of observations at different points, Capt. Cook was 



117. Node months of Venus ? Where line of nodes ? Why June and 
December her node months 3 ( Why only one month after those of Mercury ?) 

118. When last transit of Venus? Next? (How many have been ob- 
served? What said of Kittenhouse? Webster * When next four transits 
of Veims ? When first transit noticed ? What said of it ? That of 1769 ? 
Cook — use of observations ?) 



6i 



astronomx. 



sent to the Pacific in the bark "Endeavor," where he perished subsequently by the 
hands of savages at one of the Sandwich islands. Observations upon these transits fur- 
nish data for important astronomical calculations. 

119. In consequence of the earth's annual revolution 
around the sun, he appears to travel eastward, through 
all the signs of the zodiac, every 365J days. It is this 
eastward motion of the sun that causes the stars to rise 
and set earlier and earlier every night. 

sun's apparent motion around the ecliptic. 




Let a person walk around a tree, for instance, at a sbort distance from it, and it will 
appear to sweep around the horizon in an opposite direction. So as the earth revolves 
annually about the sun, the sun appears to traverse the circle of the heavens in the oppo- 
site direction. Suppose the earth is at A on the 20th of March ; the sun will appear to 
be at B in the opposite side of the ecliptic. As the earth moves on in her orbit from A 
to C. the sun will appear to move from B to D; and will seem thus to traverse the 
whole circle of the heavens every 8654: days, or as often as the earth revolves around 
him. The time of the sun's apparent entrance into the different constellations, as he jour- 
neys eastward, is usually laid down in almanacs. Thus: "Sun enters T (Aries) 20th of 
March, etc. j" at which time the earth would enter the sign Off (Aquarius), and the sun 
would seem to enter the opposite sign Aries. 



119. What said of sun's apparent motion ? Cause ? Time of revolution ? 
Effect upon the stars? (Illustration from tree? By diagram.) What is 
meant V>v the sun's entering Aries ? When i Where earth then ? 



PRIMARY PLANETS. 65 



CHAPTER II 



PRIMARY PLANETS CONTINUED. 

120. Besides the revolution around the sun, the planets 
all revolve rapidly about their respective axes, as they 
perform their celestial journeys. This is called their 
diurnal revolution. 

The evidences of the earth's revolution have already been considered on pages 13 and 
14 That most of the other planets revolve has been ascertained by carefully observing 
the motions of spots, as they seemed to pass periodically over their disks. 

121. The axis of the earth is inclined to the plane of 
the ecliptic 23° 28'. It is always 'parallel to itself — that 
is, it always inclines the same way, and to the same 
amount. 

INCLINATION OF THE EARTHS AXIS TO THE PLANE OF THE ECLIPTIC. 
PLANE OF 0^% THE ECLIPTIC. 

1. The inclination of the earth's axis, and its parallelism to itself, are exhibited in the 
above cut, as also in the cuts, pages 50, 51, and 64, to which the student will do well to 
turn. 

2. The author is aware that the poles of the earth have a slow motion around the pole 
of the ecliptic, requiring 25,000 years for a single revolution, but prefers to consider this 
point hereafter, in connection with the precession of the equinoxes. 

122. The axes of all the planets are inclined more or 
less to the planes of their respective orbits. This incli- 
nation, so far as known, is as follows : 



Yenus . . . 75° 
Mars .... 28° 42' 



Jupiter ... 3° 04' 
Saturn ... 26° 50' 



120. What revolution have the planets besides around the sun ? What 
called ? (W T hat proof of the earth's revolution ? Of the other planets ?) 

121. What said of the axis of the earth ? Of the stability of its inclina- 
tion ? (Is there no variation ?) 

122. Are the axes of the other planets inclined ? To what extent, respect- 
ively ? (Substance of note 1 ? Illustrate by diagram. Note 2 ?) 



66 



ASTKONOMY. 



1. The student will bear in mind that the above inclination is not to the ecliptic, or 
piano of the earth's orhit,, but to the plane of tho orbits of tho several planets respect- 
ively. Take the case of Venus, for instance: 




PLANE JOFJ[ENVJ£_ORBIT 

plane: of the ecliptic 




Tho orbit of Venus departs from the ecliptic 3J°, as stated at 10S, while her axis is in- 
clined to the plane of her orhit 75°, as shown in the above figures. This distinction 
should be kept definitely in view by tho student. 

2. The inclination of tho axes of the several planets, each to the plane of its own or- 
bit, is represented In the following cut : 

INCLINATION OF TIIK AXES OF THE SEVERAL PLANETS TO TIIE TLANES OF THEIR ORBITS. 



Venus 




123. The inclination of the earth's axis to the plane of 
the ecliptic causes the equinoctial to depart 23° 28 / from 
the ecliptic. This angle made by the equinoctial and the 
elliptic is called the Obliquity of the Ecliptic. 



OBLIQUITY OF TIIE ECLIl'TIC. 

B A 

>3°2S' ' 




Let tho line A A represent the axis of the earth, and B B tho poles or axis of the eclip- 
tic. Now if the lino A A inclines toward the plane of the ecliptic, or, in other words, 
departs from the line B B to the amount of 23° 28', it is obvious that the plane of the 

123. What effect has the inclination of tho earth's axis upon the equinoc- 
tial ? What is tho obliquity of tho ecliptic? (Illustrate by diagram.) 



EQUINOCTIAL AND SOLSTITIAL POINTS. 67 



equator, or equinoctial, will depart from the ecliptic to the same amount. This depart- 
ure, shown by the angles CC, constitute the obliquity of the ecliptic. 

124. The permanent inclination of the earth's axis, and 
her revolution around the sun, cause first one pole to be 
enlightened and then the other, thus producing the sea- 
sons. The same inclination and revolution cause the sun 
to appear to oscillate from north to south, crossing the 
equator twice every year. This is called the surfs decli- 
nation. (See page 26.) 

This subject of the seasons will be sufficiently understood by examining the cuts on 
pages 64 and G5. 

125. The equinoctial points in the earth's orbit are two 
points in opposite sides of the ecliptic, at which the sun 
is exactly in the equinoctial; or, in other words, the 
plane of the equinoctial exactly cuts the sun's center. 
The first of these is passed on the 20th of March (the 
sun beginning then to decline northw r ard), on account of 
w^hich it is called the vernal equinox / and the other on 
the 23d of September, on account of which it is called 
the autumnal equinox. (See the earth at A and 13, in 
the cut, page 64.) 

If the sun is vertical at the equator, he will, of course, shine to both poles, as repre- 
sented in the cut, and the days and nights will be equal all over the world. Hence the 
name equinoctial, from the Latin cequus, equal, and nvx, night. 

126. The solstitial points are those points in the earth's 
orbit w^here the sun ceases to decline from the equinoc- 
tial, and begins again to return toward it. They are 
respectively 90° from the equinoctial points. 

The Summer Solstice is reached on the 21st of June, 
when the sun has the greatest northern declination, and 
it is summer in the northern hemisj>here. 

The Winter Solstice is reached on the 23d of Decem- 
ber, when the sun has the greatest southern declination, 
and it is summer in the southern hemisphere, and winter 
in the northern. (See the earth at E F, cut, page 64.) 

124. What other effects from the inclination of the earth's axis ? Sun's 
declination ? 

125. What are the equinoctial points? How distinguished, and why? 
When passed ? (Substance of note V\ 

120. The solstitial points ? How far from the equinoctial points ? How 
distinguished ? When passed ? 



68 



ASTEONOMY. 



SHADOWS AT THE EQUATOR. 



127. The amount of the sun's declination north and 
south of the equinoctial is 23° 28' ; answering to the in- 
clination of the earth's axis, by which it is caused, and 
marking the limits of the tropics upon the earth's surface. 

1. On the 21st of June the sun reaches his greatest northern declination, or Summer 
Solstice, and is vertical on the Tropic of Cancer. From this time he approaches the 
equator of the heavens till the 20th of September, when he crosses it, and* begins to de- 
cline southward. On the 23d of December he has 

reached his greatest soutliern declination, or Winter 
Solstice, and begins to return toward the equinoctial, 
which he possesion the 20th of March, and reaches his 
Summer Solstice again on the 21st of June. In this 
manner he continues to decline, first north and then 
south of the equator, from year to year. But it should 
not be forgotten that the sun does not really move, 
first north and then south, but that the apparent mo- 
tion is caused simply by the inclination of the earth's 
axis and her revolution around the sun. 

2. The sun's declination may be easily measured 
by the shadow of a suitable object upon the earth's 
surface. Suppose the flag-staff" in the cut to stand 
perpendicularly, and exactly on the equator. On 
the 23d of December the shadow would be thrown C JJ A 
northward to A, or 23° 2S'— just as far as the sun has 

declined south. At 12 o'clock, on the 20th of March, and the 23d of September, there 
would be no shadow; and on the 21st of June, it would extend southward 23° 28' to 
C. Thus, at the equator, the shadow falls first north and then south of all perpendicular 
objects, for six months alternately. 



/■ 


Ik 


9-/ : 
ff 


'■A 


/ 




/ 23°28' |f 23°28' \ 



MEASURING THE SUN S DECLINATION IN NORTHERN LATITUDE?, 



"■-, .""••'->* 



*i** 




.?e c 



S3. 



^&^&i^ss^^ 



"^^^tif^fTT — 




3. This cut shows how the student may measure the sun's declination wherever ho 
may be located north of the equator. The shadows are such as are cast by objects 
during the year, about 45° north of the equator. On the 23d of December, when the 
sun has his greatest declination, the shadow of the flag-staff extends north at 12 
o'clock to the point C, where two boys are seen, having just driven down a stake. 
From this time to June 21st the shadow gradually shortens, till on that day it reaches 
the point B, where another stake is driven. It then begins to elongate, and in six 
months is extended to C again. The point A is just half-way from B to C in angular 
measurement, though the^distances on the plain in the picture are very different. 
When the sun is on"the equator, March 21st and September 23d, the shadow will reach 
only to A; and the angle A B and the top of the staff shows the northern, and A C and 
the top of the staff the southern declination. It will be found to be 23° 2S' each way, 
as marked in the figure. 



127. To what extent does the sun decline from the equinoctial north find 
south? Why not more? (Substance of note 1 ? Note 2, and explain by- 
diagram. Note 3, and diagram. What is a gnomon ?) 



EOTATJON OF THE PLANKTS UPON THEIR AXES. 



69 



4. The angle formed by the top and bottom of the pole and the point A will exactly 
correspond with the latitude of the place where the experiment is made. 

5. Let the students try this matter for themselves. Select a level spot, and put up a 
stake, say ten feet high. Get an exact " noon mark," or north and south line, where 
the stake is driven, and at 12 o'clock, every fair day, put down a small stake at the end 
of the shadow. In this manner you will soon be able to measure the sun's declination 
for yourselves, to determine the latitude of the place where you live, and to understand 
how mariners at sea ascertain their latitude by the declination of the sun. 

6. The ancients had pillars erected for the purpose of making observations upon their 
shadows. Such a pillar is called a gnomon. 

ROTATION OF THE PLANETS UPON THEIR AXES. 

128. The time, so far as known, of the revolution of 
the planets upon their respective axes, or, in other words, 
the length of their natural days, is as follows : 





h. m. 




h. m. 


Mercury . . 


. 24 5 


Juno . . 


. . 27 


Venus . . 


. 23 21 


Jupiter 


. . 9 56 


Earth . . . 


. 24 00 


Saturn 


. . 10 29 


Mars . . . 


. 24 37 


Uranus 


. . 9 30 



These statistics are given upon the authority of Sir John F. W. Herschel, though he 
marks Juno and Uranus as doubtful. 

129. The revolution of the earth upon its axis is the 
cause of the agreeable vicissitudes of day and night. 



PHILOSOPHY OF DAY AND NIGHT. 





How wisely adapted to the happiness of His creatures are all the works of God! Tho 
night prepares us for the day, and the day in turn prepares us to welcome the night; 
and in both instances the change ministers to the happiness of man and beast. Andbut 
for being carried around into the darkness of the earth's shadow, we should never have 
admired the dazzling firmament, as it declared the glory of God, and showed forth his 
handiwork. How beautiful the poetic allusion to the revealing power of night ! 

Mysterious Night ! when our first parent knew 

Thee, from report divine, and heard thy name, 

Did he not tremble for this lovely frame, 
This glorious canopy of light and blue ? 
Yet, 'neath a curtain of translucent dew, 

Bathed in the rays of the great setting flame, 

Hesperus with the host of heaven came ; 
And lo ! creation widen'd in men's view. 



128. In what time do the other planets rotate on their respective exos ? 
(Note ?) 

129. Cause of day and night? (Substance of note ? Poetic quotation ?) 



70 ASTRONOMY. 



Who could have thought such darkness lay conceal'd 

"Within thy beams, O Sun ! or who could find, 
Whilst fly, and leaf, and insect stood revealed, 

That to such countless orbs thou mad'st us blind ? 
Why do we, then, shun death with anxious strife ? 
If light can thus deceive us, may not life ? 

130. The earth and all the other »™^*™™ 10 * ov 

lliiii Jr .LAIN Hi IS. 

planets revolve eastward upon their 
axes, or in the same direction that they 
revolve in their orbits. This also is 
determined (with the exception of the 
earth) by observing the motion of spots 
upon their surfaces, by the aid of tele- 
scopes. 

1. In the cut we have an arc of the earth's orbit, and the 
earth revolving on her axis as she revolves around the sun. 
The arrows show the direction in both cases. 

2. By holding the book up south of him, and looking at- 
tentively at the cut, the student will understand why the 
Bun " rises" or first appears in the east. It is because the 

earth revolves eastward. Thus the observer at A is carried round into the light, and 
Bees the sun rise when he reaches B. 




/; 



TIME. 

131. Time is duration measured either by natural or 
artificial means. The principal natural indicators of the 
lapse of duration are the revolution of the earth upon its 
axis, marking a natural day ; the change of the moon, 
denoting a lunar month ; and the cycle of the seasons, 
denoting a year. Time is measured artificially by clocks, 
watches, chronometers, dials, &c. ; the standard being 
the solar day still, which is divided artificially into 24 
parts, called hours, and these again into minutes and 
seconds. 

The aboriginal tribes of this country all reckoned time by "moons," or months, as 
denoted by the moon's changes. 

132. The motion of the earth upon its axis is the most 
regular of which we have any knowledge. It does not 
vary one second in a thousand years. 

To this stability of the earth's motion upon her axis the prophet refers when he says : 
" Thus saith the Lord, If ye can break my covenant of the day, and my covenant of the 

130. In what direction do the planets rotate on their axes ? How ascer- 
tained ? (Explain why the sun appears to rise in the east.) 

131. What is timet What natural standards ? Artificial? (How meas- 
ured by aborigines ?) 

132. What said of earth's motion on axis ? (What reference to in Scrip- 
tures ?) 



TIME. 71 



night, and that there should not be day and night in their seasons, then may also my 
covenant be broken with David," &c.~ Jeremiah xxxiii. 20. 

133. Time is of two kinds — Solar and Sidereal. A 
solar day is the time elapsing from the sun's crossing the 
meridian of any place, to his coming to the same me- 
ridian again. A sidereal day is the time intervening 
between the transit of a star across the meridian, to its 
coming to the same meridian again. 

134. A solar day consists of 24 hours, at a mean rate, 
but a sidereal day is accomplished in 23 hours, 56 min- 
utes, and 4 seconds ; the solar day being near 4 minutes 
the longest. This slight difference of about 4 minutes 
daily, between solar and sidereal time, amounts to one 
whole day in every 36 5 \. Owing to the revolution of 
the earth around the sun, and his apparent annual revo- 
lution eastward among the stars, it requires 366 revolu- 
tions of the earth, as measured by the fixed stars, to 
make 365^ days, as measured by the sun. 

135. The cause of this difference in the apparent revo- 
lutions of the sun and stars, and consequent difference in 
the length of a natural day, as measured by the passage 
of a star or of the sun across the meridian, is this : The 
earth is constantly advancing in her orbit while she re- 
volves on her axis, causing the sun to appear to move 
slowly eastward among the stars ; or, what is the same 
thing, the stars to appear to rise earlier and earlier every 
night, and one after another to overtake and pass by the. 
sun. (See Article 119.) When, therefore, the meridian 
is brought around to that point in the heavens where the 
sun was near 24 hours before, he is not there, but has 
moved a little eastward. But a star that, 24 hours before, 
was exactly behind the center of the sun in the distant 
heavens, w r ili be found west of the sun, and will conse- 
quently cross the meridian before the sun does. The 
time required for the meridian to revolve from the star 
to the sun constitutes the 3 minutes 56 seconds difference 
between solar and sidereal time. 

183. Kinds of time ? Define each. 

134. Length of solar day ? Sidereal? Difference? Amount in year ? 

135. State the cause of the difference in the time of the apparent revolution 
of the sun and stars. Illustrate by diagram. 



72 ASTRONOMY. 



80LAF. AND SIDEREAL 1IME. 
Q. SIDEREAL DAY _ 



S0LAR_5£?.. 



is^ir-;;::;:;;;.".*. ^njcwthe n^rjdjan 



1. To the man at A the sun (S) is exactly on the ?2ieridian, or it is twelve o'clock, 
noon. The earth passes on from B to D, and at the same time revolves on her axis. 
When she reaches D, the man who has stood on the same meridian has made a complete 
revolution, as determined by the star G (which was also on his meridian at twelve o'clock 
the day before) ; but the sun is now east of the meridian, and he must wait four minutes 
for the earth to roll a little further eastward, and bring the sun again over his north and 
south line. If the earth was not revolving around the sun, her solar and sidereal days 
would be the same ; but as it is, she has to perform a little more than one complete revo- 
lution each solar day, to bring the sun on the meridian. 

EQUATION OF TIME. 

136. As the distant stars have no motion, real or ap- 
parent, around the ecliptic, and the earth's motion upon 
it is uniform, it results that sidereal time is always exactly 
the same. 

A clock that keeps sidereal time is called a sidereal clock. One of these instruments 
is almost indispensable in the observatory of the astronomer. 

137. Solar time is constantly varying. 'No two suc- 
cessive solar days are exactly of a length. The 24 hours 
given as the length of a solar day (134:) is the average 
of all the solar days throughout the year. Hence it is 
called mean solar time. The time, as indicated by the 
transit of the sun across the meridian, from day to day, 
is called apparent time. 

138. A well-regulated clock will "keep mean solar time, 
and will vary from the apparent time (as indicated by a 
noon mark, or dial) to the amount of 16 \ minutes one 
way, and 14J the other. The sun will at one time cross 
the meridian 1Q\ minutes before it is noon by the clock — 
the apparent time being 16 \ minutes faster than mean or 
clock time ; while at another time it will be noon by the 
clock 14J minutes before it is noon by the sun. 

186. Is sidereal time always the same ? Why must it be ? (What is a 
sidereal clock ?) 

137. What said of the variations of solar time ? What is mean solar time f 
Apparent ? 

138. What time do common clocks keep ? How much variation from sun 2 
How ? 



EQUATION OF TIME. 73 



139. The difference between apparent and mean solar 
time is called the Equation of Time. It is greatest about 
the 3d of November, when the clock is 16 minutes and 
17 seconds behind the sun. Four times a year — viz., 
April 15th, June 15th, September 1st, and December 23d 
— the clock and sun will agree; or, in other words, mean 
and apparent time will be alike. 

140. The inequality of the solar days depends upon 
two causes — the unequal velocity of the earth in her 
orbit (77, 78), and the inclination of her axis to the plane 
of her orbit (123). 

141. If the earth's orbit were an exact circle, she 
would move with the same EQUAL BO lar days. 
velocity in all parts of it; m 
and if she revolved with m(^ 
regularity upon her axis, j& M 9 
her solar days would be f®^ 
exactly of a length. ^ 

Let the circle in the adjoining cut rep- : ^#fe. 

resent the earth's orbit, and the projec- »j&— # fli 



#-^.#' 



tions from the earth toward the sun a ; ^f, m 

'pillar or gnomon standing upon a given 
meridian. The cut will then show that 
with a circular orbit, and uniform motion 
in it, and a regular rotation upon her 
axis, ihe earth would bring the gnomon 
around toward the sun at regular inter- 
vals, both of distance in her orbit, and 
of time. In that case, all apparent solar 
days would be equal. 

142. As the orbit of the earth is elliptical, it requires 
more time for the earth to pass from the vernal equinox, 
through the aphelion, to the autumnal equinox, than it 
does from the autumnal equinox, through the perihelion, 
to the vernal equinox. The difference is about eight days 
— the sun being north of the equinoctial about eight days 
longer than he is south of it. Hence the summers of the 
northern hemisphere are longer than the winters. 

143. As the earth's orbit is an ellipse, and the earth 

139. "What this difference called ? When greatest? When no difference ? 

140. What causes the inequality in the length of the solar days ? 

141. What necessary in order that they may be equal ? (Illustrate by dia- 
gram and explanations.) 

142. What effect has the ellipticity of the earth's orbit upon the length of 
the seasons, north and south of the equator ? 

4 



74 ASTRONOMY. 



moves faster in some parts of it than in others, while its 
rotary motion is uniform, it follows that its orbitual ve- 
locity in longitude must 
sometimes be faster, and 
at others slower than its 
orbitual motion , thus caus- g 

ing an inequality in the ^ ' *^J 

length of the solar days. ^ j$ 



UNEQUAL SOLAR DAYS. 
D 



J$ 



From A to B in the adjoining cut, 
the orbitual motion is slower than its 
mean rate, and the rotary motion gains dpfa 

upon it. Hence the gnomon is shown B^— ^ #* 

revolving too fast, and as pointing east ^\ v40jfcJ 

of the sun, when the earth has per- L -~sL 

formed her journey for a mean solar w/~~ r 1 

day. From B to A, the earth's motion \ ..•' 

in her orbit gains upon her rotary mo- y £^ ffi 

tion, and the gnomon is behind, or west \ y 

of the sun. At A and B the clock %v *^' ^ * €&'' 

and sun would agree. From A to D *--.. j^L ** 

the sun gains on the clock, till it gets *'--*sr"' 

14£ minutes ahead. From D to B this C 

difference is diminished, till at B the 

sun and clock agree. From B to C the clock gains on the sun, till the difference is 16$ 

minutes ; and from C to A this difference diminishes, till at A mean and apparent time 

agree again. 

144. The earth's perihelion is in II, and her aphelion 
in t ; the first of which she passes on the first of Janu- 
ary, and the latter on the 3d of July. "We are conse- 
quently about three millions of miles nearer the sun 
Jan. 1, than July 3d. 

The natural effect of this variation would be, so far as it had any influence, to modify 
the cold and heat in the Northern Hemisphere, and to augment both in the Southern. 
For instance, our nearness to the sun in January would slightly soften our winter, while, 
at the same time, it slightly increased the heat of the summer south of the equator. 
So, also, our increased distance in July would diminish the heat of our summer, and at 
the same time enhance the cold of the corresponding winter in the Southern Hemi- 
sphere. But the variation of 3,000,000 miles is so slight, when compared with the whole 
distance of the sun, that the change of temperature produced thereby is imperceptible. 

THE CALENDAR, LEAP YEAR, OLD AND NEW STYLE, ETC. 

145. The Julian calendar divided the year into 12 
months, containing in all 365 days. But a full astro- 
nomical year, or the time requisite for the earth to re- 
volve from one equinox around to the same equinox 
again, consists of 365d. 5h. 48m. 51s. Hence the Julian 

143. Explain the cause of this inequality ? (Illustrate by diagram.) 

144. Where are the perihelion and aphelion points, and when passed ? 
When nearest, and how much ? ( What effect?) 

145. Describe the Julian calendar? An astronomical year? What dif- 
ference ? What effect ? How corrected ? 



THE CALENDAR, LEAP YEAR, ETC. 75 



year was near 6 hours, or one day in every four years, 
too short; which, if left uncorrected, would in time com- 
pletely reverse the seasons, giving harvests in January, 
and snow in July. To prevent this constant falling be- 
hind, a correction was applied, by adding one day to 
February every fourth year. Hence it is called Bissex- 
tile or Leap Year. 

146. But one whole day added for every four years 
was 44m. 36s. too much. From a. d. 325 to 1582 this 
excess amounted to about 10 days ; so that the civil year 
was thus much ahead of the astronomical. In 1582, 
Pope Gregory XIII. applied a further correction, or re- 
formed the Julian calendar. To make the civil and as- 
tronomical years agree, so that the vernal equinox would 
happen on the 21st of March, as it did 1257 years before, 
Gregory resolved to strike out of the civil year the 10 
days it had gained, and ordered that the 5th of October 
should be called the 15th. This reformed or corrected 
calendar is called the Gregorian calendar. 

147. To prevent the civil year from running ahead of 
the astronomical again, in the lapse of centuries, by the 
11m. 12s. which it exceeded the astronomical, it was pre- 
scribed that at certain convenient periods the intercalary 
day of the Julian period should be omitted. Thus the 
centennial years 1700, 1800, 1900, are, according to the 
Julian calendar, bissextiles ; but on these it was ordered 
that the intercalary day should not be inserted, inserted 
again in 2000, but not inserted in 2100, 2200, 2300 ; and 
so on for succeeding centuries. 

148. The Gregorian or reformed calendar was adopted 
as soon as promulgated, in all Catholic countries ; but in 
England, the " change of style, 55 as it was called, did not 
take place till September, 1752. Eleven nominal days 
were then struck out, and the 3d of September was called 
the 14th. At the same time, the time of the beginning 

146. Was the calendar then correct ? Why not ? What result ? Who 
corrected ? W T hen ? How ? What this reformed calendar called ? 

147. What further correction necessary ? How effected ? 

148. Was the Gregorian calendar at once adopted? When in England? 
How then adopted ? What other change at the same time ? What effect in 



76 



ASTRONOMY. 



of the civil year was changed from the 25th of March to 
January 1st, as it now stands. The year 1752, which 
was to have begun on the 25th of March, w^as made to 
begin on the 1st of January preceding ; so that for dates 
falling between the 1st of January and the 25th of March, 
the number of the year is one greater by the New than 
by the Old Style. And as the intercalary day was omit- 
ted in 1800, there is now, for all dates, 12 days difference 
between the old and new styles. Russia is now the only 
Christian country in which the Gregorian calendar is not 
used. 



TIME, AS AFFECTED BY LONGITUDE. 

149. As the sun's crossing the meridian of any place 
determines it to be 12 o'clock, apparent solar time, at 
that place, it is evident 




, t - 

lyiiliF 1 



that 12 o'clock comes 
sooner to places east on 
the earth's surface, and 
later to places west. 

1. Let the adjoining cut represent the 
earth, the arrows indicating the direc- 
tion of her revolution, and the sun being 
on the meridian at XII. at the top. It 
will then be day over all the light por- 
tion of the globe, and night over all the 
shaded portion. On the meridian 
exactly under the sun it is just XII. 
o'clock noon ; while at the meridian on 
the opposite side of the earth it is just 
12 o'clock at night, or midnight. When 
the light and shade meet on the right, it 
is VI. o'clock morning; and directly 
opposite on the left, is VI. o'clock even- 
ing. 

2. Observe that when it is XII. at A, 
it is I. o'clock at B, II. o'clock at C, &c, 

while it is only XL o'clock at D, X. o'clock at E, IX. o'clock at F, &c. ; thus showing 
how it is that time is earlier east, and later west of any given meridian. 

150. Every 15° of longitude upon the earth's surface 
makes an hour's difference in the time. If east of the 
given meridian, it will be an hour earlier ; if west, an 
hour later. 



reckoning years of time ? What the difference now between Old and New 
style, and why ? What calendar used in Kussia ? 

149. What effect lias the longitude of a place upon its time ? (Diagram, 
and explain ?) 

150. What difference of longitude is required to make an hour's difference 
in time? When earlier? When later? (How demonstrated? When 6 



TIME, AS AFFECTED BY LONGITUDE. 77 



1. If the sun passes through 360° every 24 hours, he must pass over 15° each hour, 
as 360° -f- 24= 15°. llcnceevery 15° must make an hour's difference in the time ; and 
when it is sunrise, or <) o'clock, solar time, in New York city, it will be noon, or 12 
o'clock, 90° east of Now York, and midnight 90° west of it. 

2. Taking the circumference of the earth at 25,000 miles, the sun passes over 1041 ^ 
miles every hour at the equator; for 25.000 miles -4-24 hours =1041§ miles. And if 
1041 smiles be divided by 60, the number of minutes in an hour, it gives about 1 74 miles 
as the space over which the sun travels at the equator every minute. Every 17£ miles, 
therefore, castor west. Mill make one minute's difference in the time. As we recede 
ftom the equator north or south, the meridians approach each other, and a degree of 
longitude becomes less and less to the poles. 

3. A person leaving Boston with the exact time will find, on reaching Albany, about 3° 
west of Boston, that his watch is some 12 minutes ahead of the Albany time; and on 
reaching Buffalo, about 5° further west, that it is some 32 minutes ahead of the true time 
at Buffalo. So in traveling from Buffalo to Boston, the Albany and Boston time will be 
found to be the same extent ahead of the Buffalo time. Hence conductors on railroads, 
running their trains by time, set their watches from Albany to Buffalo by some standard 
agreed upon — as, for instance, Syracuse time— and reject all other local time, be it foster 
or slower. - 

151. As every 15° upon the earth's surface makes an 
hour's difference in the time, it is easy to convert degrees 
into time, or time into degrees. By this means, a mari- 
ner having the time at the place whence he sailed, and 
the time where he is, from observing when the sun 
crosses the meridian, can ascertain, from the difference 
between his standard and local time, his distance east or 
w r est of the port whence he sailed, or, in other words, his 
longitude. 

1. Time is converted into degrees by multiplying the hours by 15 for the degrees, and 
adding one-fourth of the minutes to the product; for every minute of time makes ^°, 
end every second of time -\' in longitude. 

2. On the other hand, degrees of longitude are converted into time by dividing them 
by 15 for the hours, and multiplying the remainder, if any, by 4 for the minutes, &c. 

152. The rotation of the planets upon their respective 
axes has caused them to swell out at their equators, and 
contract at their poles — thus assuming the form of oblate 
spheroids (page 18). 

1. When fluids are left free to yield to the influence of attraction, as mutually existing 
between their particles, they invariably assume a spherical form. Hence water, in fall- 
ing from the clouds, takes the form of spherical drops; and melted lead, thrown from 
the top of a shot-tower, takes a spherical form, and cooling in the air on its passage 
down, remains perfect little globes, called shot. 

2. A solid sphere would never become oblate by revolution. It might burst, from 
its powerful centrifugal tendency, as grindstones sometimes do in manufactories of cut- 
lery; but it must be Jluid, or at least soft and yielding, in order to become oblate by 
revolution. 

o'clock in New York, what time 90° east ? — 90° west ? How many miles does 
the sun pass over in an hour at the equator? Per minute ? *How deter- 
mined ? How north and south of equator ? At 45th degree ? What differ- 
ence from Boston to Albany and Buffalo ? From Buffalo to Boston ? Heneo 
what practice ?) 

151. (Jan time be converted into degrees, and degrees into time? How 
useful in navigation ? (How convert time into degrees ? Degrees into 
time ?) 

152. Effect of rotation upon figure of planets? (Note 1 ? 2. -Solids? 
o. What does oblateness indicate ? 4. Proof from Scriptures ? Remark?) 



78 



ASTRONOMY. 



3. The oblateneSS of the planets, ilicn, seems to indicate two things: First, that they 
Were all once in a fluid or plastic states ; and, secondly, that they began to revolve while 
in that state, or before any part of them had become solid, like OUT continents and 
islands. 

\. So far as the earth is concerned, Ave are taught in the Holy Scriptures— the best 
and most accurate of all hooks — that the earth and water of our globe were once so 
mixed, that, the whole appeared as a " void" of u waters ;" and that they were afterward 
separated into "earth" and "seas" by the Almighty Creator. (See Genesis i., 2, 0, lit.) 
Thus we see that true science and the Bible are always in harmony with each other. 

153. The difference between the polar and equatorial 
diameters of the planets, so far as known, is as follows : 
the Earth, 26 miles; Mars, 25; Jupiter, G,000; and 
Saturn, 7,500. 

The oblatcness of Jupiter and Saturn is as plainly risible through a telescope, as the 
difference in the following figures is to the eye of the student. 



ORIGINAL FORM. 



CHANGE. 



PRESENT AIM'KAUANOE. 





The plain line in the middle figure shows the original form, and the dotted line its 
present form. The difference is the change produced by its rotation. When measured 
by the proper instruments, it is found, in the case of Jupiter, to amount to about ,\- 
of his average diameter; and that being 89,000 miles, ^ is but little less than G,000. 

154. As Mercury and Venus rotate in about the same 
time of our globe, and their sidereal years are only 88 
and 225 days respectively (71), it follows that Mercury 
has but 88 natural days to his year, and Venus only 
about 225 to hers. But the natural day of Jupiter being 
only 10 hours long, and his year equal to about 12 of 
ours (11 years 317 days), he must have 10,307 natural 
days in one of his years. So Saturn's year, consisting of 
29 years 175 days of our time, will allow him to rotate 
on his axis about 25,000 times; or, in other words, will 
allow of 25,000 natural days in each of his years. The 
year of Uranus being equal to 84 years and 27 days of 



IT);'. State the difference of equatorial and polar diameters of planets? 
(Remark respecting Jupiter and Saturn?) 

l")4. ]Iow many natural dayshaa Mercury in his year? Venus? Jupiter? 
How so many I Saturn ? Uranus I (Demonstrate.) 



TIME, AS AFFECTED BY LONGITUDE. 79 



our time (71), and his diurnal revolution 9^ hours (128), 
it follows that he has 92,683 natural days in his year. 

29 years 175 days = 10,760 days of our time; X24 = 25S,240 hours -f- 10i- hours, the 
time of Saturn's revolution, =24,594^, the number of days in his year. So 84 years, 
27 days, the periodic time of Uranus = 36,6S7 days, or SS0.4S8 hours ; which ~»£ hours, 
the time of the planet's diurnal revolutions 92,0S3, the number of natural days in his 
year. 

155. As going from the earth's center is to ascend 
(page 27), and the equator of an oblate spheroid is fur- 
ther from the center than the poles, it follows, that the 
earth being . an oblate spheroid, we must ascend some- 
what in going from either pole to the equator. A river, 
therefore, running for a great distance toward the equator, 
would actually ascend ; or, in other words, run up hill 
— the centrifugal force generated by the earth's motion 
driving the water on toward the equator. 

The Mississippi is said to be higher at its mouth than it is some thousands of miles 
north of it If its bed conforms at all to the general figure of the earth, this must cer- 
tainly be the case, as may be demonstrated by the aid WAT ™ prvvivft VT , nTTT 
of the annexed diagram. Let A B represent the polar, ATE " ■"™» LP niLL - 
and C D the equatorial diameters. The entire differ- y 
ence between them is 26 miles, or 13 miles on each ..^-s^-'^'-^z:^ 
side. The two circles represent this difference. Now jfr "x* 
as the earth's circumference is 25,000 miles, the dis- * V 
tance from the poles to the equator (being one- / \ 
fourth of that distance) must be 6.250 miles; and En / ^ 
that 6.250 miles the ascent is 13 miles, or over two / \ 

miles to every 1,000 toward the equator. The Mis- j^J \ j-B 

sissippi runs from the 50th to the 30th degree of I 

north latitude inclusive, or 21 degrees; which, at \ 

69 £ miles to a degree, would amount to about 1,500 \ 

miles. If, then, it runs a distance equivalent to 1,500 ^ 

miles directly south (in a winding course of about 

3,000), theory requires that it should be about three >^_ 

miles higher at its mouth than it is 1.500 miles directly 

north There is some philosophy, therefore, in saying 

that if a river runs for a great distance from either pole toward the equator, it must run 

up hill. 

156. Should the earth cease to rotate upon its axis, the 
waters about the equator would at once rush toward the 
poles, flooding them to the depth of 6^ miles, and reced- 
ing from the equator to the same amount. So far as the 
solid portions of the earth would permit, it would at once 
become a perfect sphere. (See page 17, and also Art. 153 
and note.) 

157. It has already been stated (77), that the orbits of 
all the planets were ellipses ; but they are not all alike 
eccentric. The orbit of Mercury is quite elliptical, while 

155. What curious fact follows from the earth's oblateness? (What in- 
stance griven ? Illustrate by diagram.) 

156. What would be the effect should the earth cease to rotate ? 



80 



ASTRONOMY 



that of Venus is nearly a circle. The student should 
observe that the eccentricity is not the deviation from a 
circle, but the distance from the center of an ellipse to 
either foci (see page 23 and cuts). 

The eccentricity of the orbits of the principal planets is as follows: 






Miles. 

Mercury 7,000.000 

Venus 492,000 

Earth 1,618,000 

Mars . . .' 13,500,000 

Vesta 21,000,000 

Astraea 

Juno 64,000,000 



Miles. 

Ceres 21.000,000 

Pallas : 64,250,000 

Jupiter 24,000,000 

Saturn 49,000,000 

Uranus 85,000,000 

Neptune 



PRECESSION OF THE EQUINOXES. 



PRECESSION OF THE EQUINOXES. 

158. The equinoctial points have already been defined 
(125) as two points in the earth's orbit where the equi- 
noctial or celestial equator (20) cuts the sun's center. 
They are in opposite sides of the ecliptic, or 180° apart 
(see 119 and cut). The vernal equinox is the point from 
which both celestial longitude and right ascension are reck- 
oned (20 and 91) ; but not being marked by any fixed ob- 
ject in the heavens, it is reached just when the sun comes 
to be exactly over the earth's equator, or in the equinoctial. 

159. But it is 
found by long and 
careful observation 
that the earth reaches 
the equinoctial point 
about 22 minutes 
and 23 seconds ear- 
lier every year than 
on the year preced- 
ing. This is equal 
to 50}" of arc iii the 
ecliptic. In this 
manner the equinoc- 
tial points are slowly 

westward, 



receding 




157. What said of the orbits of Mercury and Venus ? Of eccentricity? 

158. Are the equinoctial points marked by any fixed object in the heavens ? 
How know when reached ? 

159. Are they stationary or not ? Reached how much earlier annually ? 



PRECESSION OF THE EQUINOXES. SI 



or falling back upon the ecliptic, at the rate of 50J" a 
year, or 1° every 71f years. This would amount to 30°, 
or one whole sign in 2,140 years, and to the entire circle 

of the ecliptic in 25,868 years. 

This very interesting phenomenon may be explained by the preceding diagram. Let 
the point A represent the vernal equinox", reached, for instance, at 12 o'clock on the 20th 
of March. The next year the snn will be in the equinoctial 22 minutes 23 seconds ear- 
lier, at which time the earth will be 50!" on the ecliptic, back of the point where the 
sun was in the equinoctial the year before. The next year the same will occur again; 
and thus the equinoctial point will recede westward little by little, as shown by the small 
lines from A to B. and from C to D. It is in reference to the stars going forward, or 
seeming to precede the equinoxes, that the phenomenon was called the Precession of 
the Equinoxes. But in reference to the motion of the equinoxes themselves, it is rather 
a recession. 

160. The cause of this wonderful motion was unknown, 
until Xewton proved that it was a necessary consequence 
of the rotation of the earth, combined with its elliptical 
figure, and the unequal attraction of the sun and moon 
on its polar and equatorial regions. There being more 
matter about the earth's equator than at the poles, the 
former is more strongly attracted than the latter, which 
causes a slight gyratory or wabbling motion of the poles 
of the earth around those of the ecliptic, like the pin of 
a top about its center of motion, when it spins a little 
obliquely to the base. 

161. One marked effect of this recession of the equi- 
noxes is an increase of longitude in all the heavenly 
bodies. As the vernal equinox is the zero or starting 
point, if that recedes westward, it increases the distance 
between it and all bodies east of it ; or, in other words, 
increases their longitude to the amount of its recession. 
Hence catalogues of stars, and maps, showing their lon- 
gitude, need to be corrected at least every 50 years, 
otherwise their longitude, as laid down, will be too little 
to indicate their true position. Allowing the world to 
have stood at this date (1853) 5,857 years, the equinoxes 
have receded already through about 75° of longitude. 
At the same time the constellations have gone forward 

Hew much in angular measurement ? Revolving which way ? At what 
rate ? How long for l c i For 80° ? For the whole circle of the ecliptic ? 
Instrate by diagram.) 

160. Cause of recession ? Who discovered? 

161. Effect of recession upon longitude \ Explain how effected. Signs 
and constellations ? 

4* 



82 



ASTRONOMY. 




eastward, and left the signs which bear their names. 
Hence the sign Aries actually covers the constellation 
Pisces. 

162. Another effect of the recession of the equinoxes 
is, that it gives to the pole of the earth a corresponding 
revolution around the 

pole of the ecliptic in 
25,868 years. 

Let the line A A in the figure 
represent the plane of the ecliptic; 
B B, the poles of the ecliptic ; C 0, 
the poles of the earth ; and D D, 
the equinoctial. E E is the obliquity 
of the ecliptic. The star C at the 
top represents the pole star, and the 
curve line passing to the right from 
it may represent the circular orbit 
of the north pole of the heavens 
around the north pole of the ecliptic. 

163. This gyratory 
motion of the north 
pole of the heavens, 
while it keeps at the 
distance of 23° 28' from 
the pole of the ecliptic, will cause it to change its place 
in the heavens to the amount of 46° 56' in 12,934 years; 
thus alternately approaching toward and receding from 
the stars, at every revolution of the equinoxes around the 
ecliptic. Thus the place of the pole is in constant but 
very slow motion around the pole of the ecliptic. 

164. The Nutation of the earth's axis is another small 
and slow gyratory motion, by which, if subsisting alone, 
the pole would describe among the stars, in the period of 
about 19 years, a minute ellipse, having its longer axis 
equal to IS", and its shorter about 14" ; the longer axis 
pointing toward the pole of the ecliptic. It is on account 
of these varied motions shifting the point from which 
longitude and right ascension are reckoned, and also the 
pole of the heavens, that it becomes necessary, in de- 

162. What other effect of recession ? (Illustrate by diagram.) 

163. What effect upon the apparent distance of the stars from the north 
pole of the heavens ? 

164. What is Nutation? What meant by epoch, and why necessary to 
state ? 



TELESCOPIC VIEWS OP THE PLANETS — MERCURY. 83 



scribing the place of a star or planet, by any of these 
standards, to state the epoch or time, and also whether it 
be mean right ascension — i. £., right ascension after hav- 
ing been corrected for the recession of the equinox, the 
zero point. 

165. The Colures are two great circles crossing at the 
poles of the ecliptic at right angles. One passes through 
the equinoxes, and is thence called the Equinoctial 
Cohere / the other passes through the solstices, and is 
called the Solstitial Colure. They are to the heavens 
what four meridians, each 90° apart, would be to the 
earth. 



CHAPTER III. 



TELESCOPIC VIEWS OF THE PLANETS. 

166. By the aid of telescopes, we discover myriads of 
objects in the heavens that are entirely invisible to the 
naked eye ; while objects naturally visible are immensely 
magnified, and seem to be brought much nearer the ob- 
server. 

This impression of nearness is an intellectual conclusion drawn from the fact of the 
increased distinctness of the object; as we judge of the distance of objects, in a great 
measure, by their dimness or distinctness. 

MERCURY. 

167. Under favorable circumstances, Mercury is visible 
to the naked eye, but yet is seldom seen, owing to his 
nearness to the sun. During a few days in March and 
April, and August and September, he may be seen for 
several minutes in the mornino; or evening twilight, when 

165. What are the colures ? Describe. 

166. Effect of the telescope upon vision ? Upon distant objects ? (Why- 
appear nearer ?) 

167. Can Mercury be seen by the naked eye? Is he often seen? Why 
not ? When may he be seen? How appear? 



84 ASTRONOMY. 



his greatest elongations (99) happen in those months. 
He appears like a star of the third magnitude, with a 
pale rosy light. See 104 and note. 

168. Through a telescope, Mercury exhibits different 
phases in different parts of his orbit, similar to those pre- 
sented by the moon in her revolution around the earth. 
The German astronomer, Schroeter, discovered numerous 
mountains upon the surface of Mercury, one of which he 
estimated to be nearly 11 miles in hight. By observing- 
these at different times, he determined the diurnal revo- 
lution of the planet to be 24h. 5m. 28s. But these observa- 
tions have not been confirmed by any other astronomer. 
The apparent angular diameter of Mercury varies from 
5" to 12", according to his position with respect to the 
earth (56 and 80). So far as is known, he is not attended 
by any satellite. 

VENUS. 

169. When favorably situated, Venus is one of the 
most conspicuous members of the planetary system, and 
is a most brilliant object even to the naked eye. Her 
color is of a silvery white, and, when at a distance from 
the sun, either east or west, she is exceedingly bright and 
beautiful. When nearest the earth, her apparent di- 
ameter is 61", which is greater than that of any other 
planet, owing to her being so much nearer than Jupiter 
or Saturn. 

Under a telescope, Yenus exhibits all the phases of 
the moon, as she revolves around the sun. The cause of 
this phenomenon is, that we see more of her enlightened 
side at one time than at another ; and the same is true 
of Mercuiy. 

1. The telescopic appearance of Venus, at different points in her orbit, is represented 
in the following figure. At E and W she has her greatest eastern and western elonga- 

168. How appear through telescope? What said of Schroeter ? What 
conclusion from observing the spots ? Confirmed by others, or not ? An- 
gular diameter of Mercury ? Wny vary ? Has he a satellite ? 

169. What said of Venus ? Her apparent diameter? Why greater than 
that of Jupiter ? How appear through telescope ? Cause of her phases ? 
(Describe phases when east of the sun — west. What prediction before 
the discovery of the telescope ?) 



TELESCOPIC VIEWS OF VENUS. 



85 



tion. and is stationary; while her positions opposite the words "direct" and "retro- 
grade" represent her at her conjunctions. The spots on the face of the sun represent 
Venus projected upon his disk, in a transit, the arrow indicating her direction. 



TELESCOPIC PHASES OF VENUS. 




^^^^^^^^^j^gf^^tg^f^^^g^^ 


BIB 


"-^-T^^gg^ 


IM^T^T^h •&' -''i "■ - 


■pi 


Fi^m3l33CXlI 


j 



2. Before the discovery of the telescope, it was asserted that if the Copernican theory 
were true. Mercury and Venus would exhibit different phases at different times ; and as 
those phases could* not be seen, it, was evident that the theory was false. But no sooner 
had Galileo directed his small telescopes to these objects,. than he found them exhibiting 
the very appearances required by the Copernican theory, its opponents themselves being 
judges. 

170. Besides the phases above mentioned, a close in- 
spection of Venus will reveal a variety of spots upon 
her surface. These are supposed to be the natural divi- 
sions of her surface, as continents, islands, &c. Schroeter 
measured several mountains upon this planet, one of 
which he estimated at over twenty miles in hight. There 
is evidence of the existence of an atmosphere about this 
planet, extending to the distance of about three miles. 

SPOTS SEEN ETON THE SURFACE OF VENUS. 




171. Were a person situated upon one of the exterior 
planets, at a distance from our globe, it would exhibit 
phases like Mercury and Venus, in its annual revolution ; 
and the continents, islands, and seas would appear only 
as spots upon her surface, assuming various forms, ac- 
cording to the position from which they were viewed. 

170. What else seen upon Venus ? What supposed to be ? , Schroeter's 
measurements \ Has Venus an atmosphere ? 

171. How would our globe appear it* viewed from a distance ? 



86 



ASTRONOMY. 



DISTANT TELESCOPIC VIEWS OF THE EARTH. 

i. 2. a 4. 




Above we have four different views of our own globe. No. 1 is a view of the 
Northern Hemisphere; No. 2, of the Southern ; No. 3, of the Eastern Continent; No. 4, 
of the Western. A common terrestrial globe will present a different aspect from every 
new position from which it is viewed ; as the earth must in her appearance to the in- 
habitants of other worlds. 

MARS. 

172. Mars usually appears like a star of the second 
magnitude, of a reddish hue. When in opposition, or 
nearest to the earth, he appears quite brilliant, as we see 
his disk fully illuminated. His apparent diameter is 
then about 18" ; whereas, when on the opposite side of 
the ecliptic, or in conjunction with the sun (80), it is only 
4". He exhibits slight phases, and his surface seems to 
be variegated with hill and vale, like the other planetary 
bodies. " Upon this planet," says Dr. Herschel, " we 
discern, with perfect distinctness, the outlines of what 
may be continents and seas." When it is winter at his 
north pole, that part of the planet is white, as if covered 
with ice and snow ; but as summer returns to his north- 
ern hemisphere, the brightness about his north pole dis- 
appears. 

173. The general ruddy color of Mars is supposed by 
Sir John Herschel to indicate " an ochery tinge in the 
general soil, like what the red sandstone districts on the 
earth may possibly oifer to the inhabitants of Mars." 
Others suppose it to indicate the existence of a very 
dense atmosphere, which analyzes the light reflected from 
the planet. 

When the sunlight passes through vapor or clouds in the morning or evening, the 
different rays of which it is composed are separated, and the red rays only pass to tho 



172. Usual appearance of Mars ? When brightest, and why? Apparent 
diameter ? Cause of great variation ? Phases ? Herschel's remark ? Spot 
at north pole ? 

173. Supposed causes of his color ? (Note.) 



THE ASTEROIDS. 87 



earth, giving to the clouds a gorgeous crimson appearance. In a similar manner it is 
supposed that the atmosphere'of Mars may give him his crimson hue. 



TELESCOPIC APPEARANCES OF MARS. 




1. The right-hand figure represents Mars as seen at the Cincinnati Observatory, 
August 5, 1S45. On the 30th of the same month he appeared as represented on the left. 
The middle view is from a drawing by Dr. Dick. 

2. Just east of the " Seven Stars," or Pleiades, the student will find another group 
called the Jlyades ; one of which, called Aldebaran, is of a reddish cast, and somewhat 
resembles the planet Mars. AY hen Mars is in opposition, however, at his nearest point 
to us, and with his enlightened side toward us, he appears much larger and brighter than 
Aldebaran. 

174. As the periodic time of Mars is only 1 yr. 322 
days (71), his motion eastward among the stars will be 
very rapid, as in that time he must traverse the whole 
circle of the heavens. His rate of motion being about 
1° for every two days, or one whole sign in 57 days, it 
will be easy to detect his eastward progress by observing 
his change of position with reference to the fixed stars, 
for a few evenings only ; and by marking his place occa- 
sionally for two years, we may track him quite around 
the heavens! 

THE ASTEROIDS. 

175. The Asteroids are invisible except through tele- 
scopes, though Testa was once seen by Schroeter with 
the naked eye. Few of them present any sensible disks, 
even under the telescope. They have a pale ash-color, 
with the exception of Ceres, which is of a reddish hue, 
resembling Mars. A thin haze or nebulous envelope has 
been observed around Pallas, supposed to indicate an 
extensive atmosphere ; but no spots or other phenomena 
have ever been detected. 

"On such planets," says Sir John Herschel, "giants might exist; and those animals 
which on earth require the buoyant power of water to counteract their weight might 

174. What said of the eastward motion of Mars ? How detected ? Rate ? 

175. Are the asteroids visible to naked eye ? Schroeter ? How appear 
under telescope ? Ceres ? Pallas ? (Remarks of Sir John Herschel ?) 



88 



ASTRONOMY. 



there be denizens of the land. A man placed on one of these planets might spring with 
ease to the night of 60 feet, and sustain no greater shock in his descent than he does on 
the earth from leaping a yard. 1 ' See 05 to 67", and notes. 



TELESCOPIC VIEW OP JUPITER. 



JUPITER. 

176. To the naked eye, Jupiter appears like a fine 
bright star of the first magnitude. His apparent di- 
ameter varies from 30" to 46", according to his distance 
from the earth. His color is of a pale yellow. Under a 
telescope, his ob- 
1 a ten ess is plainly 
perceptible (as 
shown at 135), 
and his disk is 
seen to be streak- 
ed with curious 
belts, running 
parallel to his 
equator, as shown 
in the cut. 

1. The number of belts 
to be seen upon the disk 
of Jupiter depends very 
much upon the power of 
the instrument through 
which he is viewed. An 
ordinary telescope will 

show the two main belts, one each side of his equator; but those of greater power ex- 
hibit more of these curious appendages. Dr. Herschel once saw his whole disk covered 
with small belts. 

177. These belts sometimes continue without change 
for months, and at other times break up and change their 
forms in a few hours. They are quite irregular, both in 

form and apparent density / as both bright and dark 
spots appear in them, and their edges are always broken 
and uneven. They are supposed by some to be openings 
in the atmosphere of the planet, through which its real 
body is seen; while others think they may be clouds, 
thrown into parallel strata by the rapid motion of Jupi- 
ter upon his axis. The spots in the belts are thought to 

176. Jupiter to naked eye? Apparent magnitude? Cause of variation ? 
Color ? Figure ? Belts ? (Number of belts ? Ordinarily ? As seen by 
Herschel ? 'What view in the cut ?) 

177. Are these belts permanent and regular? What supposed to be? 
What said of spots in the belts? What 'ascertained by observing spots? 
(What said in note ?) 




SATURlSr. 



89 



be caverns or mountains, or, at least, something perma- 
nently attached to the body of the planet. It was by 
watching these that the rotation of the planet upon his 
axis was ascertained. 

One of these spots, first observed in 16G5, disappeared, and reappeared regularly in the 
same form for more than forty years ; showing conclusively that it was something per- 
manent, and not a mere atmospherical phenomenon. 

178. In examining Jupiter w T ith a telescope, from one 
to four small stars will be seen near him, which, on 
examination, will be found to accompany him in his 
eastward journey around the heavens, and to revolve 
statedly around him. These are the moons of Jupiter, 
of which we shall speak more fully under the head of 
Secondary Planets. 

The writer once saw all four of these satellites at once, and very distinctly, through 
a common ship telescope, worth only twelve or fifteen dollars. They were first seen 
bv Galileo with a telescope, the object-glass of which was only one inch in diameter! 
If the student can get hold of any sucirinstrument whatever, let him try it upon Jupi- 
ter, and see if he cannot see from one to four small stars near him, that will occupy 
different positions at different times. 

179. As the periodic time of Jupiter is 11 years 317 
days (71), his rate of motion eastward through the fixed 
stars is about 30° a year. Still, this motion can soon be 
detected, and in 12 years we may w T atch his progress 
quite around the heavens. 

The writer has watched this planet from the constellation Aries, west of the "Seven 
Stars," till he passed that group, and onward through » , n, ©, &c, to njj, his present po- 
sition (1S53). In five years (185S) he will get around to Aries again, where he was seen in 
IS46; and thenceforward will perform the same journey again every twelve years. 

SATURN. 

180. This planet is plainly visible to the naked eye, 
appearing like a star of the third magnitude, of a pale 
bluish tint. His average angular diameter is about 18". 
By the aid of the telescope, he is found not only to be 
oblate, and striped with belts, and attended by satellites 
like Jupiter, but to be encircled by a suite of gorgeous 
rings, which renders him one of the most interesting 
objects in all the heavens. 

178. What else discovered about Jupiter ? What are they ? (Remark in 
note ?) 

179. Jupiter's rate of motion eastward ? Is it easily detected ? (Remark 
in note? Where was Jupiter in 184:6 ? In 1853? Where now f Where in 
1S58 

180. Natural appearance of Saturn? Angular diameter? Appearance 
through telescope : 



90 



ASTRONOMY. 



181. The oblateness of Saturn (15) is distinctly visi- 
ble through good telescopes (as shown in the cut), while 
the body of the planet is of a lead color, and the rings 
of a silvery white. They may be compared to concen- 
tric circles (18) cut out of a sheet of tin. They are 
broad, flat, and thin, and are placed one within the other 
directly over the equator of the planet, and revolve with 
him about his axis, in the same direction, and in the 
same time (128). They are estimated to be about 100 
miles in thickness. 

182. These rings are solid matter, like the body of the 
planet. This is proved by the fact that they sometimes 
cast a strong shadow themselves upon the body of the 
planet, and at other times show the planet's shadow very 
distinctly upon their own surfaces. It is also evident 
that they are wholly detached from the jplanet y &s the 
fixed stars in the distant heavens beyond have been seen 
through the opening in the rings, and between the planet 
and the first ring. 

The adjoining cut is an ex- 
cellent representation of Sa- 
turn as seen through a tele- 
scope. The oblateness of the 
planet is easily perceptible, 
and his shadow can be seen 
upon the rings back of the 
planet. Theshadow of the 
rings may also be seen run- 
ning across his disk. The 
writer has often seen the 
opening between the body of 
the planet and the interior 
ring as distinctly as it appears 
to the student in the cut. Un- 
der very powerful telescopes, 

these rings are found to be again subdivided into an indefinite number of concentric 
circles, one within the other, though this is considered doubtful by Sir John Herschel. 

183. As our view of the rings of Saturn is generally 
an oblique one, they generally appear elliptical, and 
never circular. The ellipse seems to contract for about 
7-2~ years, till it almost entirely disappears, when it begins 



TELESCOPIC VIEW OF SATURN. 




181. Oblateness? Color? Kings— what like ? How situated? What 
motion ? Thickness ? 

182. What said of substance of the rings ? What proof? "What evidence 
that they are detached? (Remark of author as to seeing satellites? Re- 
specting rings ? Opinion of Uerschcl ?) 

180. What the general apparent figure of the rings ? Why elliptical ? 



SATURN. 



91 



to expand again, and continues to enlarge for 7£ years, 
when" it reaches its maximum of expansion, and again 
begins to contract. For fifteen years, the part of the 
rings toward us seems to be thrown up, while for the 
next fifteen it appears to drop below the apparent center 
of the planet ; and while shifting from one extreme to 
the other, the rings become almost invisible, appearing 
only as a faint line of light running from the planet in 
opposite directions. The rings vary also in their inclina- 
tion, sometimes dipping to the right, and at others to the 
left. 

TELESCOPIC PIIA8ES OP THE RINGS OF SATURN. 




The above is a good representation of the various inclinations and degrees of expan- 
sion of the rings of Saturn, during his periodic journey of 30 years. 

184. The rings of the planet are always directed more 
or less toward the earth, 



PERPENDICULAR VIEW OF TITE RINGS OF SATURN. 



and sometimes exactly 
toward us ; so that we 
never see them perpen- 
dicularly, but always 
either exactly edge- 
wise, or obliquely, as 
shown in the last figure. 
"Were either pole of the 
planet exactly toward 
us, we should then have 
a perpendicular view of 
the rings, as showm in 
the adjoining cut. 

185. The various phases of Saturn's rings are ex- 
plained by the facts that his axis remains parallel to it- 
self (see following cut), with a uniform inclination to the 




What periodic variation of expansion ? Of inclination ? When nearly in- 
visible \ 

184. How are the rings situated with respect to the earth? How would 
they appear if either pole of Saturn were toward us ! 



92 



ASTRONOMY 



plane of his orbit (123), which is very near the ecHptic 
(108); and as the rings revolve over his equator, and at 
right angles with his axis, they also remain parallel to 
themselves. The revolution of the planet about the earth 
every 80 years (72) must therefore bring first one side 
of the rings to view, and then the other — causing all the 
variations of expansion, position, and inclination which 
the rings present. 




I, Here i Rrst, that the axis of Saturn, like those of all the other pi 

remains permanent, i S . and as the vines are in the plane o( his 

or, and at ri^ht ancles with his axis, they also must remain parallel to theim 
whatever position the planet may oeenpy in its orbit. 

-. Tins being the ease, it is obvious that while the planet Is rom \ to I 

SOU Will shine upon •. Side of the rings; ami while he p:issos from B 

tt \ again, upon the - de: and as it requires about 3(1 years tor the 

planet to traverse these two semicircles, it is plain that the alternate day and night on 
the rings \\ 111 he 15 years each. 

3 Land Bare and C and Inta in the orl 

At A and K the ringsai - the stm. and also toward the I 

provided Saturn rain rver on the earth, tin 

seem I nltoU and to 'rem C to mi K to * 

center, and from K to A . i low it, 

tS, 1'rom tin 
miary\1$4& lh* came to Ins equinox Septemhei which time to 

will continue to expand. From that time to June, 1S68, they 

II reach his other equ . and the rings will he in> 

i June, 1863, to September. 1870, thej will again expand; and from September, 
:>77. they will contract, when he will be at the equinox passed Soptem- 

trs before, 
i writer has often seen the rl am in different stages of exp. 

raction, and - an the] were almost directly < s d the ear; 

thai time (January, LS49), the) appeared as a bright line of light, as represented 



. What - >f these varying phases, a Ixplain by dia- 
gram, When rings invisible ! When at his equinox ! U rings ex- 
pand I When ringa next invisible; l'\ . I Con- 
\ . what point then! Author's obsen 



SATURN. 93 



186. The dimensions of the rings of Saturn may be 
stated in round numbers as follows : 

Miles. 

Distance from the body of the planet to the 

first ring 19,000 

Width of interior ring 17,000 

Space between the interior and exterior rings 2,000 

Width of exterior ring 10,500 

Thickness of the r»gS 100 

Those statistics, as given by Sir John Herschel, arc as follows: 

Exterior diameter of exterior ring.. 40"*095 ss 170,418 miles. 

Interior do 85"-289 = 155,272 « 

Exterior diameter of interior ring 84"*475 = 151,690 " 

[nterior do 90"'668 = 117,889 " 

Equatorial diameter of the body 17" "991 = 79,100 " 

Interval between the planet and interior ring 4"*889s= 19,090 " 

Interval of the rings 0"-40S = 1,791 " 

Thickness of the rings not exceeding '250* " 

187. The rings of Saturn serve as reflectors to reflect 
the light of the sun upon his disk, as our moon reflects 
the light to the earth. In his nocturnal sky, they must 
appear like two gorgeous arches of light, bright as the 
full moon, and spanning the 
whole heavens like a stupen- 
dous rainbow. 

In the annexed nit, the beholder is supposed 
to ho situated some 'M)° north of the equator of 
Saturn, and looking directly south. The shad* 
oir of the planet is seen travelling Up the arch 
as the night advances, while a New Moon is 
Shown in the west, and a Full Moon, in the east 
at the same time. 

L88. The two rings united are nearly 13 times as wide 
as the diameter of the moon; and the nearest is only 
j2yth as far from the planet as the moon is from us. 

1. The two rings united are '27.51)0 mileswide; which -f- 2,100 the moon's diame- 
ter =12^, So 240,001) miles, the moon's distance -r- 19,000 the distance of Saturn's in- 
terior ring= 12 j |. 

'_'. At the distance of only 19,000 miles, our moon would appear some forty times as 
large ;i> sin- docs at. her present distance. How magnificent and Inconceivably grand. 
then, must these vast rings' appear, witli a thousand times the moon's magnitude, and 
only one-twelfth part of her distance ! 

186. stnte the distances and jjiraensiona of tois rings, beginning at t lie body 
of the planet, and passing outward. (What additional statistics from Her- 

el ?) 

187. What purpose do the rings of Saturn servo? How appear in hifl 
evening sky ? 

188. Width of two rings, us compared with moon ? Distance ? (Demon- 
strate both. How would our moon appear at the distance of Saturn's rings?) 



NIGHT 8CENE UPON BATUIIN. 



94 ASTRONOMY. 



189. Besides the magnificent rings already described, 
the telescope reveals eight satellites or moons, revolving 
around Saturn. But these are seen only with good in- 
struments, and under favorable circumstances. 

On one occasion, the writer saw five of them at once, with a six-inch refractor manu- 
factured by Mr. Henry Fitz, of New York ; but the remaining three he has never seen. 
For a further description of these satellites, see chapters on the Secondary Planets. 

190. The periodic time of Saturn being nearly 30 
years (72), his motion eastward among the stars must be 
very slow, amounting to only 12° a year, or one sign in 
2^ years. It will be easy, therefore, having once ascer- 
tained his position, to watch his slow progress eastward 
year after year. Saturn is now (October, 1852) about 
15° west of the seven stars, and consequently will pass 
them eastward early in 1854. 

URANUS. 

191. Uranus is scarcely ever visible except through a 
telescope ; and even then we see nothing but a small 
round uniformly illuminated disk, without rings, belts, 
or discernible spots. His apparent diameter is about 
4", from which he never varies much, owing to the small- 
ness of our orbit in comparison with his own. 

Sir John Herschel says he is without discernible spots, and yet in his tables he lays 
down the time of the planet's rotation (which could only be ascertained by the rotation 
of spots upon the planet 1 s disk), at 9^ hours (i2S). This time is probably given on the 
authority of Schrueter, and is marked as doubtful by Dr. Herschel. 

192. The motion of Uranus in longitude is still slower 
than that of Saturn. His periodic time being 84 years 
27 days, his eastward motion can amount to only about 
4° 11' in a whole year. To detect this motion requires 
instruments and close observations. At this elate (1853) 
Uranus has passed over about ~ of his orbit, since his 
discovery in 1781 ; and in 1865 will have traversed the 
whole circuit of the heavens, and reached the point 
where Herschel found him 84 years before. 

189. What else seen about Saturn ? When seen ? (Observations of the 
author.) 

190. Motion of Saturn eastward ? Eate ? 

191. How Uranus seen? How appear through telescopes? Apparent 
diameter ? Why so small, when so much larger than Venus ? Why so little 
variation ? (Remark respecting spots.) 

1 C J2. What said of Uranus' apparent motion? Rate per year? In 1853, 
how far since discovered ? When made a complete revolution since 1781 ? 



THE SOLAR SYSTEM IN MINIATURE. 95 



193. Uranus is attended by several satellites — four at 
least, probably five or six. 

Sir William Herschel reckoned six, though no other observer has confirmed this 
opinion; and even his son, Sir John Herschel, seems to consider the existence of six- 
satellites quite doubtful. 

NEPTUNE. 

194. Neptune is a purely telescopic planet, and his 
immense distance seems to preclude all hope of our 
coming at much knowledge of his physical state. A 
single satellite has been discoyered in attendance upon 
him, and the existence of another is suspected ; but if 
others exist, they are as yet undetected. 

195. On the 3d of October, 1846, Mr. Lassell, of 
Liverpool, England, supposed he had discovered a ring 
about the planet, similar to the rings of Saturn ; but this 
supposition has not yet been confirmed by the observa- 
tions of other astronomers. 

196. The periodic time of Neptune being 164 years 
226 days, his motion in longitude amounts to only about 
2° 10' per year ; and yet this slow motion of about 21" 
per day is easily detected, in a short time, by the aid of 
the proper instruments. It is by this motion, as well as 
by the disk which it exhibits under the telescope, that 
the object was first distinguished from the fixed stars, 
and recognized as a planet. 

THE SOLAR SYSTEM IN MINIATURE. 

197. Choose any level field or bowling-green, and in 
its center place a globe two feet in diameter, to represent 
the sun. Mercury may then be represented by a mus- 
tard-seed, at the distance of 82 feet ; Venus by a pea, at 
the distance of 142 feet ; the earth also by a pea, at the 
distance of 215 feet. A large pin's head would repre- 
sent Mars, if placed 327 feet distant ; and the Asteroids 
may be represented by grains of sand, from 500 to 600 

103. Attendants of Uranus ? How many ? (Remark in note ?) 

194. How Neptune seen? What attendant ? Suspicion? 

195. Supposition of Lassell ? Is it confirmed ? 

196. Motion of Neptune per year? W T hy so slow? Can it be detected ? 

197. What representation of the solar system? Size of sun? Mercury, 



96 ASTRONOMY. 



feet from the center. A moderate sized orange would rep- 
resent Jupiter, at the distance of 80 rods, or 1,320 feet ; 
while a smaller orange would represent Saturn, at the 
distance of 124 rods, or 2,046 feet. Place a full-sized 
cherry or small plum three-fourths of a mile distant for 
Uranus, and another a mile and a quarter distant from 
Neptune, and you have the solar system in miniature. 

198. To imitate the motions of the planets in their 
orbits, in the above illustration, Mercury must move to 
the amount of his own diameter in 41 seconds ; Venus, 
in 4m. 14s. ; the earth, in 7m. ; Mars, in 4m. 48s. ; Jupi- 
ter, in 2h. 56m.; Saturn, in 3h. 13m.; Uranus, in 2h. 
16m. ; and Neptune, in 3h. 30m. 



CHAPTER IV. 

SEASONS OF THE DIFFERENT PLANETS, ETC. 

199. The general philosophy of the seasons has already 
been explained (Art. 119 to 125). 

The inclination of the axis of a planet determines the 
extent and character of its zones ; and the length of its 
periodic time determines the length of its seasons. 

Thus the axis of the earth being inclined toward the ecliptic 23° 28', the tropics fall 
23° 28' from the equator, and the polar circles 23° 28' from the poles; and the period of 
the earth's revolution around the sun being 365^ days, it follows that each of the four 
seasons must include about three months, or 91 days on an average. If the axis was 
more inclined, the tropics would fall further from the equator, and the polar circles fur- 
ther from the poles, so that the torrid and frigid zones would be wider, and the tem- 
perate narrower; and if the earth's period was longer, her seasons, respectively, would 
be longer. 

200. The general temperature of a planet is probably 
governed by its distance from the sun (59, 60) ; but the 
temperature of any particular portion of a planet de- 
pends mainly upon the directness or obliquity with which 

and where placed ? Venus, what and where ? Earth ? Asteroids ? Mars ? 
&c. 

198. How imitate the motions of the several planets ? 

199. What determines the extent and character of a planet's zones t What 
the length of its seasons ? (Illustrate by inclination and period of the earth.) 



SEASONS OF THE DIFFERENT PLANETS, ETC. 



97 



the rays of light fall xijpon it — a circumstance that greatly 
affects the amount of light received by any given por- 
tion of its surface. Hence we have summer in the 
northern hemisphere in July, when the earth is farthest 
from the sun ; and winter in January, when she is near- 
est the sun (144). 

Though nearer the sun in January than in July, still, 
as the northern hemisphere is then inclined from the 
sun, his rays strike its surface obliquely ; less light falls 
upon the same space than if its contact was more direct, 
and it is consequently cold. But in July, the rays are 
more direct — the northern hemisphere being inclined 
toward the sun — and it is summer, notwithstanding we 
are three millions of miles further from the sun than in 
January. 



SUMMER AND WINTEK EAYS. 




1. The comparative amount of light received in the northern hemisphere in July and 
January may be illustrated by the accompany- 
ing: figure, in which the rays of light at dif- 
ferent seasons are represented to the eye. In 
January, they are seen to strike the northern 
hemisphere obliquely, and consequently the same 
amount of light is spread over a much greater sur- 
face. In July, the rays fall almost perpendicularly 
upon us, and are much more intense. Hence the 
variations of temperature which constitute the 
seasons. 

2. If the student is not perfectly clear as to how 
the north pole is turned first toward and then/rom 
the sun, he will need to be guarded against the 
vulgar idea that the earth's axis " wabbles," as it is 
called. By consulting 119 to 121, and the cuts, it 
will be seen that the very permanency of a plan- 
et's axis, combined with its periodic revolution, gives the beautiful and ever welcomo 
changes of the seasons. How simple, and yet how effectual, this Divine mechanism ! 

201. As the inclination of the axis of a planet and the 
length of its periodic time determine the extent and 
character of its zones, and the length of its seasons, it 
follows that w T here these are known, we have a reliable 
clew to the seasons of a planet, even though we have 
neither visited nor heard from it ; and as we do not know 
the inclination of the axis of Mercury, we have no 
knowledge of his seasons. 

200. What governs the general temperature of the planets ? The tem- 
perature of particular zones ? What result from this last ? Why not warm- 
est in January, &c. ? (Illustrate by diagram.) How are the poles shifted to 
and/row the sun ? Do the poles " wabble ?" 

201. How ascertain the character of the seasons of distant planets? Sea- 
sons of Mercury ? 



98 ASTRONOMY. 



202. The seasons of Venus are very remarkable. So 
great is her inclination (122), that her tropics fell within 
15° of her poles, and her polar circles (as if to retaliate 
for the trespass upon their territory), go up to within 15° 
of her equator. Thus the torrid and frigid zones over- 
lap each other, and the temperate zone is altogether 
annihilated. 

The period of Venus being but 225 days (72), the sun 
declines in that time from her equinoctial to within 15° 
of one pole ; then back to the equinoctial, and to within 
15° of the other pole, and again back to the equinoctial. 
The effect of this very great inclination is to give eight 
seasons at her equator every 225 days. 

In her short period of 225 days, the sun seems to pass from her northern solstice 
through her equinox to her southern solstice, and back to the point from which he 
started. When he is over one of her tropics, it is winter not only at the other tropic, 
but also at her equator ; and as the sun passes over from tropic to tropic, and back again 
every 225 days, making spring at the equator as he approaches it, summer as he passes 
over it, autumn as he declines from it, and winter when he reaches the tropic, it follows 
that at her equator Venus has eight seasons in one of her years, or in 225 of our days. 
Her seasons, therefore, at her equator, consist of only about four weeks of our time, or 
28| days ; and, from the heat of summer to the cold of winter, can be only about 56 
days. At her tropics, she has only Jour seasons of 56 days each. 

203. The polar inclination of Mars being 28° 40' (122), 
his torrid zone must be 57° 40' from his poles — leaving 
only 32° 40' for the width of his temperate zone. But 
as his year consists of 687 days, his four seasons must 
consist of about 172 days each, or nearly twice the 
length of the seasons of our globe. 

204. So slight is the inclination of the axis of Jupiter 
to his orbit, that he has but a narrow torrid zone, and 
small polar circles. As his orbit departs from the plane 
of the ecliptic only 1° 46' (108), and his axis is inclined 
to his orbit only 5° 3', it follows that his axis is nearly 
perpendicular to the ecliptic. The sun never departs 
more than 5° 3' from his equator ; and still, as his peri- 
odic time is about 12 years (72), he has alternately six 
years of northern and six of southern declination. His 
narrow torrid zone and small polar circles leave very ex- 

202. Seasons of Venus ? W T here her tropics ? Polar circles ? Temperate 
zone ? Sun's declination upon her ? Its effect ? (Substance of note ?) 

203. Zones of Mars ? Length of seasons, and why ? 

204. Zones of Jupiter, and why ? Describe his climate. Seasons ? Days 
and nights ? Poles ? 



SEASONS OF THE DIFFERENT PLANETS, ETC. 99 

tensive temperate zones. In passing from his equator to 
his poles, we meet every variety of climate, from the 
warmest to the coldest, with but slight variations in any 
latitude, from age to age. His days and nights are al- 
ways nearly of the same length, as the sun is always 
near his equinoctial. His poles have alternately six 
years day and six years night. 

205. The polar inclination and zones of Saturn differ 
but little from those of Mars ; but his seasons are greatly 
modified by the length of his periodic time. This being 
about 30 years, his four seasons must each be about T% 
years long ; and his polar regions must have alternately 
15 years day and 15 years night. The rings of Saturn, 
which lie in the plane of his equator, and revolve every 
10} hours, are crossed by the sun when he crosses the 
equinoctial of the planet. During the southern declina- 
tion of the sun, which lasts fifteen years, the south side 
of the rings is enlightened, and has its summer. It has 
also its day and night, by revolving in a portion of the 
planet's shadow. When the sun is at the southern tropic, 
it is midsummer on the south side of the rings, as the 
rays of light then fall most directly upon them. As the 
sun approaches the equator, the temperature decreases, 
till he crosses the equinoctial, and the long winter of fif- 
teen years begins. At the same time, the north side of 
the rings begins to have its spring ; summer ensues, and 
in turn it has fifteen years of light and heat. The influ- 
ence of these wonderful rings upon the climate of Saturn 
must be very considerable. During the winter in each 
hemisphere, they cast a deep shadow upon some portion 
of his surface during the day ; and in the summer, these 
immense reflectors so near the planet, and so bright 
in the sunlight, must contribute greatly to the light, if 
not to the w T armth, of his summer evenings. The poles 
of Saturn are alternately 15 years in the light, and 15 
years in darkness. 

206. Of the inclination of the axes of Uranus and 

205. Zones of Saturn, and why ? Length of seasons ? Kings — how en- 
lightened ? Influence upon climate ? Polar days and nights ? 



100 ASTRONOMY. 



Neptune, respectively, we have no knowledge, and con- 
sequently can form no opinion respecting their tropics, 
polar circles, zones, &c. If not too much inclined, like 
Venus, they have but four seasons in their year, which 
would make each season of Uranus 22 years and 9 days 
long, and each season of Neptune 41 years and 56 J days 
long ; as these periods are, respectively, one-fourth of the 
periodic time of the planet (72). 

Thus we see that tropics, polar circles, zones, and seasons are not peculiar to our globe, 
but are a necessary result of an inclined axis, and a revolution around the sun. Tho 
causes which produce our seasons are known to be in operation in other planetary 
worlds, and it would be unreasonable to deny that tho effect was there also. 

DISCOVERY OF THE DIFFERENT PLANETS. 

207. The old planets, as they are called, viz., Mercury, 
Venus, Mars, Jupiter, and Saturn, have been known as 
planets, or " wanderers," from the earliest ages. Uranus 
was discovered by Sir William Herschel, March 13th, 
1781. Neptune was demonstrated to exist before it had 
been seen, by M. Le Verrier, of France, August, 1816; 
and first seen by Dr. Galle, of Berlin, Sept. 23, 1846. 

208. The discovery of Neptune is probably one of the 
greatest achievements of mathematical science ever 
recorded. By comparing the true places of Uranus 
with the places assigned by the tables, it was found that 
he was not where his known rate of motion required 
him to be ; and after making all due allowance for the 
attraction of Jupiter and Saturn (65), by which pertur- 
bations would be produced, it was found that there was 
evidently the effect of some other body, exterior to the 
orbit of Uranus, the attraction of which body helped to 
cause the perturbations of Uranus. From this effect, 
produced by an unknown and invisible world, lying far 
out beyond the supposed boundaries of the solar system, 
not only was the existence of its cause demonstrated, but 
its direction, distance, mass, and period were proxi- 
mately ascertained. 

206. What said of the seasons of Uranus and Neptune ? Probable length 
of former ? Latter ? (Remark in note ?) 

207. What said of the " old planets ?" Of Uranus ? Neptune ? 

208. Describe the discovery of Neptune. Perturbations ? Tables, &c. ? 
(Describe successive steps in detail, what said of Mr. Adams ?) 



DISCOVERY OF THE DIFFERENT PLANETS. 



101 



1. On the evening of the 23d of September, 1S46, Dr. Galle. one of the astronomers 
of the Royal Observatory at Berlin, received a letter from Le Terrier, of Paris, request- 
ing him to employ the great telescope at his command in searching for the supposed 
new planet, and giving its position, as ascertained by calculation, as 825° 52-8' of geocen- 
tric longitude. Dr. Galle, taking advantage of the very evening on which he received 
Le Terrier's letter, soon discovered an object resembling a star of the eighth magnitude, 
near the spot indicated by Le Terrier, as the place of tile new planet On consulting an 
accurate star chart, it was found that no such star was there laid down, and observations 
were at once commenced, with a view to detecting any change of place. In three hours 
time, it was seen to have moved ; and by the next evening at eight o'clock, i was found 
to have retrograded more than four seconds of time (see 97 and cut) — a circumstance 
which proved it to be much nearer the earth than the fixed stars, and consequently a 
planet — the very planet which -had caused the unaccountable irregularities of Uranus. 
The geocentric "longitude of the planet, at midnight, September 23^1846, was 325°52'8'; 
which was less than 1° from the place assigned to it by Le Terrier ! The reason why 
Le Terrier wrote to Dr. Galle was, that the former had no suitable telescope for con- 
ducting the search in which he was so deeply interested. 

2. It is worthy of remark that Mr. Adams, of St. John's College, Cambridge. Eng- 
land, had' also calculated the place, &c, of the new planet, and had arrived at results 
similar to those reached by Le Terrier; but as the latter had published his conclusions 
first, the honor of the discovery is generally accorded to Le Terrier. 

209. The Asteroids have all been discoTered during 
the present century, and most of them since 1847. And 
to the number now known, it is not improbable that 
others will be added from time to time. 

The following table will show the date, &c., of the discovery of the several asteroids. 
They are laid down in the order of discovery : 



Planet. Date. 

Ceres January 

Pallas March 

Juno September 

Testa March 

Astrsea December 

Hebe July 

Iris August 

Flora October 

Metis April 

Hygeia " 

Pprthenope May 

Clio September 

Egeria November 

Irene May 

Eunomia July 

Melpomene June 

Anonymous* August 



1, 1801.... 


uiscovcrers. 

Piazzi, of Palermo. 


28, 1S02.... 


Olbers, of Bremen. 


1, 1804.... 


Harding, of Bremen. 


29. 1807.... 


. . .Olbers, of Bremen. 


8, 1845.... 


Encke, of Dresden. 


5, 1847.... 


CC tl 


13, 1847.... 


Hind, of London. 


18, 1847.... 


U It 


25, 1848.... 


Graham, of Sligo, Ireland. 


4 


Gasparis, of Naples, Italy. 


11 1850.... 


(C t< 


13, 1850.... 


Hind, of London. 


2, 1850.... 


Gasparis. of Naples. 


19, 1851.... 


Hind, of London. 


29, 1851.... 


Gasparis, of Naples. 


24, 1852.... 


Hind, of London. 


22, 1852.... 


u a 



* At the date of this writing, it is only forty days since this last planet was discov 
ered; hence it has no name as yet. The same fact accounts for the absence of the last 
two of this list from most of the preceding tables, 



209. What said of the discovery of the asteroids ? Are there probably 
others ? 



102 ASTRONOMY. 



CHAPTER V. 

SECONDARY PLANETS THE MOON. 

210. The Secondary Planets are those that revolve 
statedly around the primaries, and accompany them in 
their periodical journeys around the sun. Of these, the 
earth has one; Jupiter, four; Saturn, eight; Uranus, 
six; and Neptune, one — in all twenty. Besides these, 
there is a strong suspicion among astronomers that Venus 
is attended by a satellite, and that ^Neptune has at least 
two, instead of one. 

Sir John Herschel says Uranus is attended " certainly by four, and perhaps by six ; 
and Neptune by two or more." Outlines, Art. 533. In regard to Venus, Prof. Hind, of 
London, says: "Astronomers are by no means satisfied whether Venus should be at- 
tended by a satellite or not. * * * It is a question of great interest, and must re- 
main open for future discussion." 

211. Though the secondary planets have a compound 
motion, and revolve both around the sun and around 
their respective primaries, they are subject to the same 
general laws of gravitation — of centripetal and centrif- 
ugal force — by which their primaries are governed. 
Like them, they receive their light and heat from the 
sun, and revolve periodically in their orbits, and on their 
respective axes. In the economy of nature, they seem 
to serve as so many mirrors to reflect the sun's light upon 
superior worlds, when their sides are turned away from 
a more direct illumination. 

The design of all the secondaries may be inferred from what is said of the purposes 
for which our own satellite was created. " And God said, Let there be lights in the fir- 
mament of heaven, to divide the day from the night: and let them be for signs, and for 
seasons, and for days and years ; and let them be for lights in the firmament of heaven, 
to give light upon the earth: and it was so. And God made two great lights; the 
greater light to rule the day, and the lesser light to rule the night: he madethe stars 
also."— Gen. i., 14—16. 

210. What are the Secondary planets ? How many ? How distributed ? 
What supposition respecting Venus ? Neptune ? (Herschel's remark ? Prof. 
Hind's ?) 

211. What said of the laws by which the primaries are governed ? Light 
and heat ? Uses ? (From what may we infer their design ?) 



THE MOON. 103 



212. To the inhabitants of our globe, the earth's satel- 
lite or moon is one of the most interesting objects in all 
the heavens. Her nearness to the earth, and consequent 
apparent magnitude, her rapid angular motion eastward, 
her perpetual phases or changes, and the mottled appear- 
ance of her surface, even to the naked eye, all conspire 
to arrest the attention, and to awaken inquiry. Add to 
this her connection with Eclipses, and her influence in 
the production of Tides (of both of which we shall speak 
hereafter in distinct chapters), and she opens before us 
one of the most interesting fields of astronomical re- 
search. 

213. The Romans called the moon Luna, and the 
Greeks Selene. From the former, we have our English 
terms lunar and lunacy. In mythology, Selene was the 
daughter of Helios, the sun. Our English word selenog- 
raphy — a description of the moon's surface — is from 
Selene, her ancient name, and grapho, to describe. 

214. The point in the moon's apogee. 

orbit nearest the earth is called ,.... c 

Perigee, from the Greek peri, /'' *\ 

about, and ge, the earth. The point / \ 

most distant is called Apogee, from / 

apo, from, and ge, the earth. These / \ 

two points are also called the ap- \ 

sides of her orbit ; and a line join- \ 

ing them, the line of the apsides. \ 

See the moon in apogee and perigee in the cut. The 
singular of apsides is apsis. \^ /' 

215. The mean distance of the '*••-€••-" 
moon from the earth's center is, in perigee. 
round numbers, 240,000 miles; or, more accurately, 
238,650. The eccentricity of her orbit amounting to 
13,333 miles, of course her distance must vary, and also 
her apparent magnitude (56). Her average angular 

212. What said of our moon ? Why specially interesting ? 

213. Latin name of the moon ? Greek ? Derivation of words from Luna ? 
Who was Selene in mythology ? Selenography ? 

214. Perigee and Apogee 3 Derivation ? What other name for these two 
points ? What is the line of the apsides ? (Apsis ?) 

215. Moon's distance? Does it vary? Why? Eccentricity of orbit? 



104 ASTRONOMY. 



diameter is 31' 7", and her real diameter 2,160 miles. 
She is consequently only Jgth part as large as the earth, 
and y^ooVooo^ P ar t as large as the sun. 

The masses of globes are in proportion to the cubes of their diameters. Then 
2,160 X 2,160 X 2,160 = 10,077,696,000, the cube of the moon's diameter; and 7 912 
X 7,912 x 7,912 = 495,2S9,174,428, the cube of the earth's diameter. Divide the latter 
by the former, and we have 49 and a fraction over, as the number of times the bulk of 
the moon is contained in the earth. Its mass, as compared with the sun, is ascertained 
in the same manner. 

216. The plane of the moon's orbit is very near that 
of the ecliptic. It departs from the latter only about 
5J° (5° 8' 48"). 

INCLINATION OF THE MOON'S OBBIT TO THS PLANE OF THE ECLIPTIC. 




ECLIPTIC B r/~~"\ 

u 



MOTION OF THE APSIDES. 



Let the line AB represent the plane of the earth's orbit, and the line joining the 
moon at C and D would represent the inclination of the moon's orbit to that of the 
earth. At C the moon would be within the earth's orbit, and at D exterior to it ; and 
it would be Full Moon at D, and New Moon at C. 

217. The line of the apsides of the moon's orbit is not 
fixed in the ecliptic, but revolves slowly around the 
ecliptic, from west to east, 
in the period of about nine 
years. 

In the adjoining cut, an attempt is made 
to represent this motion. At A, the line 
of the apsides points directly to the right 
and left; but at B, C, and D it is seen 
changing its direction, till at E the change 
is very perceptible when compared with 
A. But the same ratio of change con- 
tinues; and at the end of a year, when the 
earth reaches A again, the line of the ap- 
sides is found to have revolved eastward 
to the dotted line IK, or about 40°. In 
nine years, the aphelion point near A will 
have made a complete revolution, and re- 
turned to its original position. 

218. The line of the moon's nodes is also in revolu- 
tion ; but it retrogrades or falls back westward, making 
the circuit of the ecliptic once in about 19 years. 

Angular diameter ? In miles ? How compare with earth ? With sun ? (How 
demonstrated ?) 

216. How is the plane of the moon's orbit situated with respect to tho 
ecliptic? (Illustrate by diagram.) 

217. Is the line of the moon's apsides stationary or not ? What motion ? 
Period ? (Illustrate.) 

218. What of the line of the moon's nodes ? In what time does it make 
the circuit of the ecliptic ? Amount of motion ? 




THE MOON. 105 



The amount of this motion is 10° 35' per annum, which would require IS years and 
£19 days ibr a complete revolution. 

219. The diameter of the moon is only ^foth . 0^ K 
part as great as that of the sun ; and yet the ajp- §®3 
parent diameter of the moon is nearly equal to '\®4&&i 
that of the sun. The former is 3r 7", and the I f 
latter 32' 2", or only 55" difference. The reason I 
why the moon appears to vie with the sun in | I 
magnitude, when she is 70 millions of times less, 1 I 
is, that she is 400 times nearer to us than he is. \ 
See Art. 56. '• j 

1. The cut in the margin will show how it is that a small object near us will '; j 
fill as large an angle, or, in other words, appear as large, as a much larger • ; 
object winch is more remote. The moon at A fills the same angle that is : j 
filled by the sun at B. • 

2. This fact may serve to illustrate the comparative influence of things '. j 
present and future upon most minds. The little moon may eclipse the sun ; ': 

or even a dime, if held near enough to the eye, will completely hide all his • j 
glories from our view. So in morals and religion. The li things which are 

seen and temporar are too apt to hide from our view the more distant but j 

superior glories of the life to come. ; I 

220. The density of the moon is only about ■ \ \ 
two-thirds that of the earth, and her surface ^ th | \ 
as great. The light reflected to the earth by her, I I 
at her full, is only 30 ovo o oth P ar ^ as much as we I j 
receive on an average from the sun. \\ 

221. The daily apparent revolution of the ^L 
moon is from east to west, with the sun and f a^\ 
stars ; but her real motion around the earth is * -' 
from west to east. Hence, when first seen as a " new 
moon," she is very near, but just east of the sun ; but 
departs further and further from him eastward, till at 
length she is seen in the east as a full moon, as the sun 
goes down in the west. 

222. The moon performs a sidereal revolution around 
the earth in 27d. 7h. 43m. ; and a synodic in 29d. 12h. 
44m. The sidereal is a complete revolution, as measured 
by a fixed star ; but the motion of the earth eastward in 

219. Moon's diameter, as compared with that of the sun? With sun's 
apparent diameter ? Why appear so near of a size ? (Illustrate by diagram. 
Reflection of the author?) 

220. Density of the moon? Her light? 

221. Her daily apparent motion ? Real motion ? How traced ? 

222. What is her sidereal revolution ? Her synodic ? What difference ? 
Why ? (Illustrate by diagram.) 

5* 



106 



ASTK0N0MY. 



her orbit gives the sun an apparent motion eastward 
among the stars (119), and renders it necessary for the 
moon to perform a little more than a complete revolution 
each month, in order to come in conjunction with the 
sun, and make a synodic revolution. 



SIDEREAL AND SYNODIC REVOLUTIONS OF THE MOON. 



* SIDEREAL REVOLUTION 27rDAYS 



(f\ 



IE 



y&*\ 



.lM"* 



\ON 



tOI 



SUN AND MOON IN CONJUNCT I N- NEW MOON 






W\ 



§d 



DAILY PROGRESS OF THE MOON EASTWARD. 



1. On the right, the earth is shown in her orhit, revolving around the sun, and the 
moon in her orbit, revolving around the earth. At A, the sun and moon are in con- 
junction, or it is New Moon. As the earth passes from D to E, the moon passes 
around from A to B, or the exact point in her orbit where she was 27£ days before. 
But she is still west of the sun, and must pass on from B to C, or 1 day and 20 hours 
longer, before she can again come in conjunction with him. This 1 day and 20 hours 
constitutes the difference between a sidereal and a synodic revolution. 

2. The student will perceive that the difference between a sidereal and synodic revo- 
lution of the moon, like that between solar and sidereal time, is due to the same cause — 
namely, the revolution of the earth around the sun. See 135. 

223. The daily angular 
motion of the moon east- 
ward is 13° 10' 35". Her 
average hourly motion is 
about 32,300 miles. This 
motion may be detected 
by watching her for a few 
hours only ; and by mark- 
ing her position, with ref- 
ence to the stars, from 
night to night, her daily 
journeys will appear pro- 
minent and striking. 

The estimate of 13° 10' 35" is made 
for a sidereal day of twenty-four hours. In the above cut, the daily progress of the 
moon may be traced from her conjunction or " change 11 at A on the right, around to the 
same point again. This being a sidereal revolution, requires only 27 £ days. 

223. Daily angular motion eastward ? How detected ? (For what day is 
this estimate made ?) 




THE MOON. 



107 



224. In her journeyings eastward, the moon often 
seems to run over and obscure 
the distant planets and stars. 
This phenomenon is called an oc- 
cupation. 



The adjoining cut represents the new moon as 
just about to obscure a distant star, by passing be- 
tween us and it. In 1850, she occulted Jupiter for 
three revolutions in succession — viz., Jan. 30th, Feb. 
27th. and March 26th. Through a telescope, the 
moon is seen to be constantly obscuring stars that 
are invisible to the naked eye. They disappear be- 
hind the moon's eastern limb, and in a short time 
reappear from behind her western ; thus distinctly 
exhibiting her eastward motion. 




225. Though the moon's orbit is an ellipse, with res- 
pect to the earth, it is, in reality, an irregular curve, 
always concave toward the sun, 



and crossing the 
every 13° nearly 



earth's orbit 



19°26:. 



1. If the earth stood still in her orbit, the 
moon would describe just such a path in the 
ecliptic as she describes with respect to the 
earth. 

2. If the earth moved but slowly on her way, 
the moon would actually retrograde on the eclip- 
tic at the time of her change" and would cross 
her own path at every revolution, as shown in 
the adjoining figure. But as the earth advances 
some 46 millions of miles, or near 100 times the 
diameter of the moon's orbit, during a single lu- 
nation, it is evident that the moon's orbit never 
can return into itself, or retrograde, as here rep- 
resented. 



B-* 



THE MOON S OIIBIT ALWAYS CONCAVE TOWARD THE SUN. 



3. That the lunar orbit is always concave toward the sun, maybe demonstrated by 
the above diagram. Let the upper curve line A B represent an arc of the earth's 
orbit, equal to that passed through by the earth during half a lunation. Now the 
radius and arc being known, it is found that the chord A B must pass more than 400,000 
miles within the earth. But as the moon departs only 240,000 from the earth, as shown 
in the figure, it follows that she must describe the curve denoted by the middle line, 
which is concave toward the sun. 



224. What are occultations f How produced ? (Are they frequent ? Are 
planets ever occulted? Describe process.) 

225. What is the form of the moon's orbit with respect to the earth ? The 
sun? (How if the earth were stationary? If moving slowly? Demon- 
strate her orbit to be concave, &c. Draw orbit for complete lunation, and 
describe her relative motion.) 



108 



ASTRONOMY. 



4. This subject may be still further illustrated by the following cut, representing 

THE MOON'S PATH DURING A COMPLETE LUNATION. 

C B 




moon's path. 



Here the plain line represents the earth's orbit, and the dotted one that of the moon. 
At A the moon crosses the earth's track 240,000 miles behind her. She gains on the 
earth, till in seven days she passes her at B as a Full Moon. Continuing to gain on 
the earth, she crosses her orbit at C, 240,000 miles ahead of her, being then at her Third- 
Quarter. From this point the earth gains upon the moon, till seven days afterward she 
overtakes her at D as a New Moon. From D to E the earth continues to gain, till at 
E the moon crosses 240,000 behind; the earth, as she had done four weeks before at A. 
Thus the moon winds her way along, first within and then without the earth ; always 
gaining upon us when outside of our orbit, and falling behind us when within it. 

5. The small circles in the cut represent the moon's orbit with respect to the earth, 
which is as regular to us as if the earth had no revolution around the sun. 

226. The moon never retrogrades on the ecliptic, or 
returns into her own path again ; but is always ad- 
vancing with the earth, at the rate 
of not less than 65,700 miles per 
hour. 

1. The moon's orbitual velocity, with respect to 
the earth, is about 2,300 miles per hour. When out- 
side the earth, as at B, in the last figure, she gains 
2,800 miles per hour, which, added to the earth's ve- 
locity, would give 70,300 miles as the hourly velocity 
of the moon. When within the earth's orbit, as at 
D, she loses 2,300 miles per hour, which, subtracted 
from 68,000 miles (the earth's hourly velocity), Mould 
leave 65,700 miles as the slowest motion of the moon 
in space, even when she is falling behind the earth. 

2. Could we look down perpendicularly upon the 
ecliptic, and see the paths of the earth and moon, 
we should see the latter pursuing her serpentine 

course, first within and then outside our globe, somewhat as represented by the dotted 
line in the annexed figure. Her path, however, would be concave toward the sun, as 
shown on the preceding page, and not convex, as we were obliged to represent it hero 
in so small a diagram. 

22T. That the moon is opake, like the rest of the plan- 
ets, and shines only by reflection, is obvious, from the 
fact that we can see only that part of her upon which 
the sun shines ; and as the enlightened portion is some- 
times toward and sometimes from us, the moon is con- 
stantly varying in her apparent form and brightness. 
These variations are called her phases. 

226. At what rate does the moon advance with the earth ? Moon's or- 
bitual velocity, with respect to the earth ? Slowest motion ? (Illustrate the 
moon's course.) 

227. What proof that the moon is opake ? What meant by her phases ? 




THE MOON. 109 



228. Tlie cause of the moon's phases — her waxing and 
waning — is her revolution around the earth, w^hich ena- 
bles us to see more of her enlightened side at one time 
than at another. 



CAUSE OF THE MOONS PIIASE8 



FIRST pR. 



i / ll \ \ om^.^ --""" 

Fyi " L ^SS^N NEW----,-""" 

3i>o -©■'- :: A : ii-c)--i • — • 



\ A P "I H / / 

'€ ^-4>--" C 

*••- ^ LAST OR. 

- c*. 

4 \ 

» 

1. This cut represents the moon revolving eastward around the earth. In the outside 
circle, she is represented as she would appear, if viewed from a direction at right angles 
with the plane of her orbit. The side toward the sun is enlightened in every case, and 
she appears like a half moon at every point. 

2. The interior suit represents her as she appears when viewed from the earth. 
At A it is New Moon ; and if seen at all so near the sun, she would appear like a dark 
globe. At B she would appear like a crescent, concave toward the east. At C, more 
of her enlightened side is visible ; at D, still more ; and at E, the enlightened hemisphere 
is fully in view. We then call her a Full Moon. From E around to A again, the dark 
portion becomes more and more visible, as the luminous part goes out of view, till she 
comes to her change at A. When at D and F, the moon is said to be gibbous. 

3. If the student will turn his book bottom upward, and hold it south of him, he will 
see why the crescent of the old moon at H is concave on the west, instead of the east, 
like the new moon, and why she is seen before sunrise, instead of just after sunset 

229. The cusps of the moon are the extremities of the 
crescent. Her syzygies are two points in her orbit 190° 
apart, where she is new and full moon. (See positions 
1 and 3 in the last cut.) The quadratures are four points 
90° apart (like 1, 2, 3, and 4 in cut) ; and her octants 
eight points 45° apart (like A, B, C, &c, in the cut). 

230. The 'moon is said to change when she comes in 
conjunction with the sun, and is changed from Old Moon 
to New Moon. 



228. Cause of phases ? (Illustrate.) 

229. What are the cusps of the moon? Her Syzygies t Quadratures ? Oc- 
tants? (Illustrate on blackboard.) 

230. What meant by the change of 'the moon? (How noticed or traced ?) 



110 



ASTRONOMY. 



If the student will be on the look-out, he can easily find the moon west of the sun 
in the daytime; and, by observing her carefully, will see that she is rapidly approach- 
ing him. In a short time she will be lost in his beams, and soon after will appear east 
of the sun, just after sundown, as a New Moon. This change, as it is called, takes place 
when she passes the sun eastward. 

231. A New Moon is the moon when she has just 
passed the sun in her eastward journey, and when only 
a small portion of her enlightened hemisphere is visible 
from the earth. She then appears like a slender cres- 
cent, concave on the east. The First Quarter is when 
she has advanced 90° eastward from the sun. She is 
then south of us at sundown, and we see one-half of her 
enlightened side. The Full of the moon is when she has 
advanced 180° from the sun, and is in the east when he 
goes down in the west. Her enlightened side is then 
toward us, and she appears circular, or full. The Third 
Quarter is when the moon has advanced 270°, or f ths 
of her synodic journey. She has been waning since the 
full, on her western limb, and is now gibbous. She is but 
90° west . of the sun, is approaching him, and waning 
more and more every day. The waxing of the moon is 
from the change to the full ; and the waning, from the 
full to the change again. 

We earnestly recommend to both teacher and student to observe the present place 
and appearance of the moon, and watch her through one lunation at least. A little time 
spent in this way will do more to fix correct ideas in the mind than months of abstract 
study. 

232. The line which separates the dark from the en- 
lightened portion of the moon's disk is called the Termi- 
nator. 

As just one-half of the moon is always enlightened by the sun, whether it appears 
so to us or not, it follows that, the terminator must extend quite around the moon, 
dividing the enlightened from the unenlightened hemisphere. This circle is called 
the Circle of Illumination. At new and full moon this circle is sidewise to us ; but 
at the first and third quarters, it is edgewise. The portion of the terminator visible 
from the earth traverses the moon's disk twice during every lunation. 

233. A variety of d^rk lines and spots may be seen 
upon the surface of the moon with the naked eye. There 
is a dark figure on her western limb, resembling that of 
a man, with his head to the north, and his body inclined 

231. What is the New Moon? How appear? First Quarter? When? 
Appearance ? Full Moon and appearance 1 Third Quarter ? Position and 
appearance ? When waxing ? Waning ? (What recommended by author ?) 

232. What is the Terminator? (Substance of note?) 

233. Describe the natural appearance of the full moon. (What said of cut ? 
Sketch on blackboard. Ojibway legend ?) 



THE. MOON NATURAL APPEARANCE. 



Ill 



to the east. Just east of him, and opposite his shorLders 
is an irregular object, resembling a huge bundle or 
pack, 

1. Both these objects are represented in the ad- natural 
joining cut, which was drawn from nature by the 
author, on the evening of December IS, 1S50. It 
represents the moon as she appears when about two 
hours high, and is the best of six different sketches 
taken during the same evening. Let the student com- 
pare it wittCthe next Fall Moon, and see if our draw- 
ing is correct. 

2. The Ojibway Indians have a legend by which 
they explain this singular appearance of the moon. 
Instead of a " man, 11 they say this figure is a beautiful 
Ojibway maiden, who was translated to the moon 
" many" snows ago.' 1 '' fur having set her affections upon 
that object, and refusing to marry any of the a young 
braves" of the Ojibway nation. How the " beautiful 
maiden'' came to look so coarse and masculine, and 
what the rest of the figure means, the tradition does 
not inform us. 

234:. These rude figures upon the moon's disk are 
probably the outlines of her great natural divisions, as 
mountains, valleys, and continents. 

Almost evervbody has noticed these rude figures upon the face of the moon, and 
manv, doubtless, have wondered what they were; but how few have supposed, as they 
were'gazins: upon her mottled disk, that they were enjoying a distant view of a icorld, 
and that these dim outlines were a nattiral map of its nearest hemisphere ! Having 
seen the "man in the moon," they have supposed it useless to pursue the subject any 
further, and here their investigations have ended. 

235. By a careful observation of the moon's disk, from 
month to month, it is found that the same side is always 
toward the earth. From this fact, it follows that she re- 
volves on her axis but once during her synodic revolu- 
tion around the earth. 




1. By watching the moon carefully with the 
naked *eye, it will be seen that the same spots 
occupy nearly the same places upon her disk 
from month* to month ; which shows that the 
same side is always toward us. 

2. Suppose a monument erected upon the 
moon's surface, so as to point toward the earth 
at New Moon, as represented at A. From the 
earth it would appear in the moon's center. 
Now if the moon so revolved upon her axis, in 
the direction of the arrows, as to keep the pillar 
pointing directly toward the earth, as shown at 
A, B, C, and D, and the intermediate points, she 
must make just one revolution on her axis during 
her periodic revolution. At A, the pillar points 
from the sun, and at C toward him ; showmg 
that, in going half way round the earth, she has 
performed half a revolution upon her axis. 



moon's revolution. 



Cf 



c 

I 
B 



« 



€> 




*4fc 



CE 



234. What are these rude figures supposed to be? (Note.) 

235. What interesting fact established by watching the moon ? 
lows from it ? (Illustrate by sketch of cut on blackboard.) 



What fol- 



112 ASTRONOMY. 



236. As the same side of the moon is always toward 
us, it follows that the earth is invisible from one-half of 
the moon. From the other half, our globe would appear 
like a stationary planet, nearly thirteen times as large as 
the moon appears to us, and exhibiting all her varying 
phases. 

237. Though the moon always presents nearly the same 
hemisphere toward the earth, it is not always precisely 
the same. Owing to the eilipticitv of her orbit, and the 
consequent inequality of her angular velocity, she ap- 
pears to roll a little on her axis, first one way and then 
the other — thus alternately revealing and hiding new 
territory, as it were,, on her 

eastern and western limbs. moon's libra™*.. 

This rolling motion east and ..••--£)--. 

west is called her libration in /'" ^ "S^. 

longitude. ^} ^£) 

The accompanying cut will illustrate the sub- / 

ject of the moon's librations in longitude. / ; 

1. From A around to C, the angular motion is : \ 
slower than the average, and the diurnal motion J^ /0*&$\ ^\ 
gains upon it, so that the pillar points west of the a V-C (fiRS/ -^\% ) 
earth, and we see more of the eastern limb of ^*/ ^$jv7 ^^^ 
the moon. \ / 

2. From C to A, again, the moon advances 

faster than a mean rate, and gains upon the MX ' D ^TS 

diurnal revolution ; so that the pillar points east ^J ^ ra) 

of the earth, and we see more of the moon's '^T* - m\ .,♦*'*' 

western limb. Thus she seems to librate or roll, Qy '" 

first one way and then the other, during every 
periodic revolution. 
At B, we see most of her eastern limb; and at D, most of her western. 

238. The axis of the moon is inclined to the plane of 
her orbit only about one and a half degrees (1° 301 10*8"). 
But this slight inclination enables us to see first one pole 
and then the other, in her revolution around the earth. 
These slight rolling motions are called her librations in 
latitude. 

As the inclination of the earth's axis brings first one pole and then the other toward 
the sun, and produces the seasons, so the inclination of the moon's axis brings first one 
pole and then the other in view from the earth. But as her inclination is only It , the 
libration in latitude is very slight. 

236. What other fact follows from the moon's keeping the same side toward 
us? How would our globe appear from the moon? 

237. What are the "moon's Vibrations? In longitude, and cause? (Illus- 
trate on blackboard.) 

238. In latitude? Cause ? (Illustrate by the case of the earth.) 



THE MOON TELESCOPIC VIEW. 



113 



239. The moon's year consists of 29^ of our days ; but 
as she makes but one revolution upon her axis in that 
time, she can have but one day and one night in her 
whole year. And so slight is the inclination of her axis 
to the plane of her orbit, that the sun's declination from 
her equator is only about 1|°. She must therefore have 
perpetual winter at her poles ; while at her equator, her 
long days are very warm, and her long nights very cold. 

240. By the aid of the telescope, the surface of the 
moon is found to be exceedingly rough and uneven, cov- 
ered with vast plains, deep valleys, and lofty mountains, 
several of the latter are from three to four and a half 
miles high. That they are really mountains is proved by 
three facts : 1st, the line of the terminator is jagged or 
uneven, as shown in the cut ; 2d., shadows are seen pro- 
jecting first to the east and then to the w r est, showing the 
existence of elevations of some sort, that intercept the 
light ; and 3d, from new to full moon, bright spots break 
out from time to time, 
just east of the ter- 
minator, in the dark 



TELESCOPIC YIEW OF THE MOON. 



and 



grow 




portion, 

larger and larger, till 
they join the illumi- 
nated portion, show- 
ing them to be the 
tops of mountains, 
which reflect the sun- 
light before it reaches 
the intervening val- 
leys. 

1. Specimens of these s7ia cl- 
ous may be seen in the cut. pro- 
jecting to the left. Bright points 
of light, or, in other words, the 
illuminated tops of mountains, 
may also be seen near the terminator, in the dark portion. The writer has often 
watched them, and seen them enlarge more and more, as the sun arose upon the side of 
the moon toward us, and enlightened the sides of her mountains. 



239. Length of moon's year ? Number of natural days ? Sun's declina- 
tion upon her ? Climate at equator and poles ? 

240. How appear through telescopes? What proof of mountains ? (Re- 
marks upon cut ? Observations of the author ? Describe shadows and their 



changes. Illustrate, by reference to the Andes and their shadow 



s.J 



114 ASTRONOMY. 



2. The shadows are always projected in a direction opposite the sun, or toward the 
dark side of the moon ; and as her eastern limb is dark from the change to the full, and 
her western from the full to the change, of course the direction of the "shadows must be 
reversed. 

3. Suppose a person stationed at a distance directly over the Andes. Before the 
Bun arose, he would see the tallest peaks enlightened; and as he arose, the long shadows 
of the mountains would extend to the west. At noon, however, little or no shadow 
would be visible; but at sunset, they would again be seen stretching away to the east. 
This is precisely the change that is seen to take place with the lunar shadows, except 
that the time required is "a lunar day, equal to about 15 of our days, instead of one of 
our days of 12 hours. 

241. Some of the lunar mountains are in extensive 
ranges, like our Alps and Andes ; while others are cir- 
cular, like the craters of huge volcanoes. Great num- 
bers of the latter may be seen with telescopes of only 
moderate power. Through such an instrument, the moon 
will appear of a yellowish hue, and the circular moun- 
tains like drops of thick oil on the surface of water. Two 
extensive ranges, and several of the circular elevations, 
are shown in the last cut. Dr. Scoresby, of Bradford, 
England, who examined the moon through the monster 
telescope of Lord Rosse, says he saw a vast number of 
extinct volcanoes, some of whose craters were several 
miles in breadth. Her general appearance was that of a 
vast ruin of nature. Dr. Herschel supposed he saw the 
light of several active volcanoes upon her surface. 

242. In regard to the existence of an atmosphere 
around the moon, astronomers are divided. From obser- 
vations during eclijDses of the sun, and other phe- 
nomena, it is thought that if the moon has any atmos- 
phere at all, it must be very limited in extent, and far 
less dense than that of the earth. Dr. Scoresby saw no 
indications of the existence of water, or of an atmos- 
phere. 

From observations during several occultations of stars, the writer is of opinion that 
a refracting medium of some sort exists in the vicinity of the moon. The atmosphere 
is doubtless subject to the general law of gravitation. Hence it is most dense at the 
earth's surface, and grows rare as we ascend. Inasmuch, therefore, as the general den- 
sity of the atmosphere of any planet is dependent upon the attracting force of that 
planet, and the moon has only about T ^d part as much attracting power as the earth, it 
follows that her atmosphere, if she has" one, ought to be much less dense than ours. 

243. That no water exists upon the moon's surface, 

241. Form of lunar mountains ? Number of craters visible ? Appearance 
of surface, as seen by Dr. Scoresby ? Dr. Herschel's supposition ? 

242. Has the moon an atmosphere ? Dr. Scoresby's statement ? (Remark 
of author ? Why moon's atmosphere must be comparatively rare '() 

243. Why thought there is no water on the moon ? 



THE MOON. 



115 



has been inferred from the fact, that it would be con- 
verted into steam or vapor during her long and hot days, 
and also from the fact that no clouds are ever seen float- 
ing around her. 

244. Professors Baer and Madler, of Berlin, have con- 
structed a map of the moon, which is characterized by 
Professor Nichol, of Glasgow, as " vastly more accurate 
than any map of the earth we can yet produce," and as 
"the only authentic and valuable work of the kind in 
existence." 

The following is a list of the principal lunar mountains, with, their hight, according 
to the recent measurements of Madler: 



Feet. Miles. 

Posidonius 19,S30 376 

Tycho 20,190 3'S3 

Calippus 20,390 3'86 

Casatus 22,810 4*32 

Newton 23,830 4'52 



Feet. Miles. 

Clavius 19,030 3-60 

Huvsrens 18,670 3'54 

Blancanus 18,010 3*41 

Movetus 18,440 3*49 



245. The apparent position of the moon in the heavens 
is one of the principal means by which mariners ascer- 
tain their longitude at sea. So regular is her motion, 
that her "place" as viewed from 
any fixed point on the earth, at 
any specified time, and with ref- 
erence to the four stars that lie in 
or near her, may be determined 
for months and vears to come ; 
and, by observing how far she ap- 
pears out of place, either east or 
west, at the time specified, we 
may determine how far we are 
east or west of the place for 
which her longitude is given in 
the tables. 

Let A in the cut represent Greenwich Observa- 
tory, near London. B is the moon, and C her appa- 
rent place among the distant stars, about 40° west of 
the star D. The ship E, having Greenwich time, as 
well as her own local time, sails from London west- 
ward ; but on observing the moon when, by Greenwich time, she ought to be at C, she 
is found to be at F, or only about 20° west of the star D. It is therefore obvious that 




244. What celebrated chart mentioned ? How characterized ? (What list 
of mountains given ? General hight ?) 

245. What use made of the moon in navigation? Explain the process. 
What called ? What other method for determining longitude ? 



116 ASTRONOMY. 



the ship is west of Greenwich, as the moon appears east of her Greenwich place. From 
this difference between her place as laid down in the tables, and her observed place, as 
referred to certain prominent stars, the mariner determines how far he is east or west of 
the meridian of Greenwich. The moon's geocentric place (or place, as viewed from the 
center of the earth) may be given instead of her Greenwich place, and the same conclu- 
sions arrived at. In either case, this is called the lunar method of determining the 
longitude. It is also ascertained by simple comparison of local and standard time, as 
explained at 151. 

246. The best time for observing the moon with a tele- 
scope is from the change to the first quarter, and from the 
third quarter to the change. Near the first and third 
quarters, the shadows of objects are seen at right angles 
with the line of vision, and to the best advantage ; while 
at full moon, objects cast no shadows visible to us. 



CHAPTER VI. 

ECLIPSES OF THE SUN AND MOON. 

247. An Eclipse is a partial or total obscuration or 
darkening of the sun or moon, by the intervention of 
some opake body. Eclipses are either solar or lunar. A 
solar eclipse is an eclipse of the sun, and a lunar eclipse 
is an eclipse of the moon. A solar eclipse is caused by 
the moon, when she passes between the earth and the 
sun, in her revolution eastward, and casts her shadow 
upon the earth. A lunar eclipse takes place when the 
moon is in opposition to the sun, and passes through a 
portion of the earth's shadow. 

The general law of shadows may be illustrated by the following: 





Here the sun and planet are represented as of the same size, and the shadow of the 
latter is in the form of a cylinder. 

246. When is the best time for viewing the moon with a telescope ? Why ? 

247. What is an eclipse ? A solar ? Lunar ? Cause of solar eclipses ? Of 
lunar ? W T hen do lunar eclipses take place ? (Illustrate the laws of shadows 
V>y diagram on blackboard.) 



ECLIPSES OF THE SUN AND MOON. 



117 




&$wp>> 



In this cut, the opalce body is the largest, and the shadow projected from it diverges, 
or grows more broad as the distance from the planet increases. 




V^ 



Here the lum inous body is the largest, and the shadow converges to a point, and takes 
the form of a cone. 




SHADOWS OF THE PLANETS. 



\ 




Here, also, the luminous body is the largest, and both precisely of the same size as in 
the cut preceding; but being placed nearer each other, the shadow is shown to be con- 
siderably shorter. 

248. All the planets, both primaries and secondaries, 
cast shadows in a direction opposite the sun (see the 
adjoining cut). The 
form and length of these 
shadows depend upon 
the comparative magni- 
tude of the sun and 
planet, and their dis- 
tance from each other. 
If the sun and a planet 
were of the same size, 
the shadow of the 
planet would be in the 
form of a cylinder, 
whatever its distance. 
If the planet was larger 
than the sun, the shad- 
ow would diverge, as 
we proceed from the planet off into space; and the 
nearer the sun, the more divergent the shadow would be. 



7€h 6*. 



Pk. 



..»"* 



s 



248. What said of the shadows of the planets ? Of their form and length ? 
How would it be if the sun and planet were of the same size ? If the planet 



118 ASTRONOMY. 



But as the planets are all much smaller than the sun, 
the shadows all converge to a point, and take the form 
of a cone / and the nearer to the sun, the shorter its 
shadow. 

These principles are partly illustrated in the preceding cut. The planets nearest the 
sun have comparatively short shadows, while those more remote extend to a great dis- 
tance. No primary, however, casts a shadow long enough to reach the next exterior 
planet. 

249. Eclipses of the sun must always happen at JVew 
Moon, and those of the moon at Full Moon. The reason 
of this is, that the moon can never be between us and 
the sun, to eclipse him, except at the time of her change, 
or new moon ; and she can never get into the earth's 
shadow, to be eclipsed herself, except when she is in op- 
position to the sun, and it is full moon. 

250. If the moon's orbit lay exactly in the plane of 
the ecliptic, she would eclipse the sun at every change, 
and be eclipsed herself at every full ; but as her orbit 
departs from the ecliptic over 5° (216), she may pass 
either above or below the sun at the time of her change, 
or above or below the earth's shadow at the time of her 
full. 

NEW AND FULL MOONS WITHOUT ECLIPSES. 
Shadow above the Earth. Above the Earth's shadow. 




Shadow below the Earth. Below the Earth's shadow. 

1. Let the line joining the earth and the sun represent the plane of the ecliptic. Now 
as the orbit of the moon departs from this plane about 5° 9', she may appear either 
above or below the sun at new moon, as represented in the figure, and her shadow may 
fall above the north pole or below the south. At such times, then, there can be no 
solar eclipse. 

2. On the right, the moon is shown at her full, both above and below the earth's 
shadow, in which case there can be no lunar eclipse. 



was largest? If brought nearer ? How if planets smallest ? How affected 
by distance ? (How, then, with planets nearest the sun ? More remote ? 
Does any primary throw its shadow out to the next exterior planet?) 

249. At what time of the moon do solar eclipses always occur? Lunar? 
Why? 

250. Why not two eclipses every lunar month? (Illustrate.) 



ECLIPSES OF THE SUN AND MOON. 



119 



LUNAR ECLIPSE. 



SOLAR ECLIPSE. 





251. Eclipses of the sun always come on from the west, 
and pass over eastward ; while eclipses of the moon come 

on from the east, and pass over 
westward. This is a necessary 
result of the eastward motion 
of the moon in her orbit. 

1. In the right hand cut, the moon is seen re- 
volving eastward, throwing her shadow upon 
the earth, and hiding the western limb of the 
sun. In some instances, however, when the 
eclipse is very slight, it may first appear on the 
northern or southern limb of the sun — that is, 
the upper or lower side ; but even then its 
direction must be from west to east. It will 
also be obvious from this figure, that the shad- 
ow of the moon upon the earth must also trav- 
erse her surface from west to east; conse- 
quently the eclipse will be visible earlier in the 
west than in the east. 

2. On the left, the moon is seen striking into 
the earth's shadow from the west, and having 
her eastern limb first obscured. By holding 
the book up south of him, the student will see 
at once why the revolution of the moon east- 
ward must cause a solar eclipse to proceed from 
west to east, and a lunar eclipse from east to 
west. To locate objects and motions correctly, 
the student should generally imagine himself 
looking to the south, as we are situated north 
of the equinoctial. The student should bear in 
mind that nearly all the cuts in the book are 
drawn to represent a view from northern lati- 
tude upon the earth. Hence by holding the 
book up south of him, the cuts will generally 
afford an accurate illustration both of the posi- 
tions and motions of the bodies represented. 

252. Eclipses can never take place, except w T hen the 
moon is near the ecliptic ; or, in other words, at or near 
one of her nodes. At all other times, she passes above 
or below the sun, and also above or below the earth's 
shadow. It is not necessary that she should be exactly 
at her node, in order that an eclipse occur. If she is 
within 17° of her node at the time of her change, she 
will eclipse the sun ; and if within 12° of her node at her 
full, she will strike into the earth's shadow, and be more 
or less eclipsed. These distances are called, respectively, 
the solar and lunar ecliptic limits. 

251. What is the direction of a solar eclipse? A lunar? Why this dif- 
ference ? 

252. Where must the moon be, with respect to the ecliptic and her nodes, 
in order to an eclipse ? What meant by ecliptic limits ? Name the distance 
of each, respectively, from the node. (Illustrate.) 





120 



ASTRONOMY. 



This subject may be understood by consulting the following figure : 

THE MOON CHANGING AT DIFFERENT DISTANCES FROM HER NODES, 









1. Let the plain line represent the ecliptic, and the dotted line the plane of the moon^ 
orbit. The light globes are the sun, and the dark ones the moon; which may be imag- 
ined as much nearer the student; hence their apparent diameter is the same. 

2. Let the point A represent the node of the moon's orbit. Now if the change occur 
when the moon is at B, she will pass beloic the sun. If when at C, she will just touch 
his lower limb. At C, she will eclipse him a little, and so on to A ; at w T hich point, if 
the change occurs, the eclipse would be central, and probably total. 

3. If the moon was at G, H, I, or J, in her orbit, when the change occurred, she would 
eclipse the upper or northern limb of the sun, according to her distance from her node 
at the time ; but if she was at K, she would pass above the sun, and would not eclipse 
him at all. The points C and J will represent the Solar Ecliptic Limits. 

253. All parts of a planet's shadow are not alike dense. 
The darkest portion is called the umbra, and the partial 
shadow the penumbra. 

UMBRA AND PENUMBRA OF THE EAETH AND MOON. 




Penumbra is from the Latin pene, almost, and umhra, a shadow. In this cut, the 
earth's umbra and penumbra will be readily found by the lettering ; while A is the um- 
bra, and BB the penumbra, of the moon. The latter is more broad than it should be, 
owing to the nearness of the sun in the cut, as it never extends to much over half the 
earth's diameter. The student will see at once that solar eclipses can be total only to 
persons within the umbra; while to all on which the penumbra falls, a portion of the 
sun's disk will be obscured. 

254. The average length of the earth's umbra is about 
860,000 miles ; and its breadth, at the distance of the 
moon, is about 6,000 miles, or three times the moon's 
diameter. 

As both the earth and moon revolve in elliptical orbits, both the above estimates are 
subject to variations. The length of the earth's umbra varies from 842,217 to 871,262 
miles ; and its diameter, where the moon passes it, varies from 5,235 to 6,365 miles. 

255. The average length of the moon's umbra is about 
239,000 miles. It varies from 221,148 to 252,638 miles, 



253. What is the umbra of the earth or moon? The penumbra f (Deriva- 
tion ? Within which are solar eclipses total ?) 

254. The average length of the earth's shadow ? 

Whv?) 



Breadth at the moonV 



distance ? (Do they vary ? 



ECLIPSES OF THE SUN AND MOON 



121 



according to the moon's distance from the sun. Its 
greatest diameter, at the distance of the earth, is 170 
miles ; but the penumbra may cover a space on the 
earth's surface 4,393 miles in diameter. 

256. When the moon but just touches the limb of the 
sun, or the umbra of the earth, it is called an ctppulse. 
(See D and G, in the first cut on the opposite page.) 

A partial eclipse is one in which only part of the sun 
or moon is obscured. A solar eclipse is partial to all 
places outside the umbra; but within the penumbra, 
where the whole disk is obscured, the eclipse is said to 
be total. A central eclipse is one taking place when the 
moon is exactly at one of her nodes. If lunar, it is 
total, as the earth's umbra is always broad enough, at 
the moon's distance, if centrally passed, to obscure her 
whole disk. But a solar eclipse may be central and not 
total, as the moon is not always of sufficient apparent 
diameter to cover the whole disk of the sun. In that 
case, the eclipse would be annular (from annulus, a 
ring), because the moon only hides the center of the sun, 
and leaves a bright ring unobscured. 



Goin£ o5*. 



ritOGKESS OF A CENTRAL ECLIPSE. 
Annular. 



Com in % ci 




257. It has already been shown (56) that the apparent 
magnitudes of bodies vary as their distances vary ; and 
as both the earth and moon revolve in elliptical orbits, it 



255. Average length of the moon's umbra ? Does it vary ? Why ? Great- 
est diameter at the earth's surface ? Of penumbra ? 

256. What is an appulse ? K partial eclipse ? A total f & central? Are 
all central eelipses total ? Why not ? What called then ? Why 3 

257. Why are some central eclipses total, and others partial and annular ? 
(Diagram.) 

6 



122 ASTRONOMY. 



follows that the moon and sun must both vary in their 
respective apparent magnitudes. Hence some central 
eclipses of the sun are total, while others are partial and 
annular. 

TOTAL AND ANNULAR ECLIPSES OF THE STJN. 

Total. 

/ I 

aJL 




1. At A, the earth is at her aphelion, and the sun being at his most distant point, will 
have his least apparent magnitude. At the same time, the moon is in perigee, and ap- 
pears larger than usual. If, therefore, she pass centrally over the sun's disk, the eclipse 
will be total. 

2. At B, this order is reversed. The earth is at her perihelion, and the moon in 
apogee ; so that the sun appears larger, and the moon smaller than usual. If, then, a 
central eclipse occur under these circumstances, the moon will not be large enough to 
eclipse the whole of the sun, but will leave a ring, apparently around herself, unob- 
«cured. Such eclipse will be annular. 

258. As the solar ecliptic's limits are further from the 
moon's nodes than the lunar, it results that we have more 
eclipses of the sun than of the moon. There may be 
seven in all in one year, viz., five solar and two lunar ; 
but the most usual number is four. There can never be 
less than two in a year ; in w^hich case, both must be of 
the sun. Eclipses both of the sun and moon recur in 
nearly the same order, and at the same intervals, at the 
expiration of a cycle of 223 lunations, or 18 years of 365 
days and 15 hours. This cycle is called the Period of 
the Eclipses. At the expiration of this time, the sun 
and the moon's nodes w T ill sustain the same relation to 
each other as at the beginning, and a new cycle of 
eclipses begins. 

259. In a total eclipse of the sun, the heavens are 
shrouded in darkness, the planets and stars become visi- 
ble, the temperature declines, the animal tribes become 
agitated, and a general gloom overspreads the landscape. 
Such were the effects of the great eclipse of 1806. In a 
lunar eclipse, the moon begins to lose a portion of her 

258. Which kind of eclipses is most frequent? Why? The greatest 
number in a year ? How many of each ? Least number, and which ? Usual 
number? What said of the order of eclipses ? Time of cycle? 

259. Describe the effects of a total eclipse of the sun. The process of a 
lunar eclipse ? 



ECLIPSES OF THE SUN AND MOON. 



123 



light and grows dim, as she enters the earth's penumbra, 
till at length she comes in contact with the umbra, and 
the real eclipse begins. 

260. In order to measure and record the extent of 
eclipses, the apparent diameters of the sun and moon 
are divided into twelve equal parts, called digits / and 
in predicting eclipses, astronomers usually state which 
"limb" of the body is to be eclipsed — the southern or 
northern — the time of the first contact, of the nearest 
approach of centers, direction, and number of digits 
eclipsed. 



FIVE DIGITS ECLIPSED. 




TWELVE DIGITS. 



urn 



^<*J&> 




"^■"^lllk^ 



261. The next annular eclipse visible in the United 
States will occur May 26, 185±. The next total eclipse 
of the sun will be August 7, 1869. 

Some of the ancients and allbarbarous nations formerly 
regarded eclipses with amazement and fear, as supernatu- 
ral events, indicating the displeasure of the gods. Colum- 
bus is said to have made a very happy use of this supersti- 
tion. When the inhabitants of St. Domingo refused to 
allow him to anchor, in 1502, or to furnish him supplies, he 
told them the Great Spirit was offended at their conduct, 
and was about to punish them. In proof, he said the 
moon would be darkened that very night / for he knew 
an eclipse was to occur. The artifice led to a speedy and 
ample supply of his w r ants. 

262. Eclipses can be calculated with the greatest pre- 
cision, not only for a few years to come, but for centuries 

260. How are eclipses measured and recorded ? 

261. When the next annular eclipse visible in this country? The next 
total \ How have the ignorant and superstitious regarded eclipses ? Anec- 
dote of Columbus ? 



124 



ASTRONOMY. 



and ages either past or to come. This fact demonstrates 
the truth of the Copernican theory, and illustrates the 
order and stability that everywhere reign throughout the 



planetary regions 



CHAPTER VII. 



SATELLITES OF T II E EXTERIOR PLANETS. 



TELESCOPIC VIEWS OF THE MOONS OF 
JUPITER. 



263. Jupiter is attended by four satellites or moons. 
They are easily seen with a common spy-glass, appear- 
ing like small stars near the 
primary. (See adjoining cut, 
and note at 178.) By watch- 
ing them for a few evenings, 
they will be seen to change 
their places, and to occupy dif- 
ferent positions. At times, 
only one or two may be seen, 
as the others are either between 
the observer and the planet, or 
beyond the primary, or eclipsed 
by his shadow. 

2G4. The size of these satel- 
lites is about the same as our 
moon, except the second, which 
is a trifle less. The first is 
about the distance of our moon ; and the others, respect 
ively, about two, three, and five times as far off. 




4th. 



COMPARATIVE DISTANCES OF JUPITER S MOONS. 

3d. 2d 1st 




2G2. What said of the calculation of eclipses ? What does this demon- 
strate and illustrate ? 

263. How many moons has Jupiter ? How seen ? Why not all seen at once ? . 
204. Their size ? Distances I Periods i Why so rapid ? 



SATELLITES OF THE EXTERIOR PLANETS. 125 



Their periods of revolution are from 1 day 18 hours to 
17 days, according to their distances. This rapid mo- 
tion is necessary, in order to counterbalance the power- 
ful centripetal force of the planet, and to keep the satel- 
lites from falling to his surface. 

The magnitudes, distances, and periods of the moons of Jupiter are as follows : 

Diameier in miles. Distance. Periodic tiiwes. 

1st 2,500 259,000 1 day IS hours. 

2d 2,068 414,000 3 " 12 " 

8d 3,377 647,000 7 " 14 " 

4th 2,S00 1,104,000 17 u " 

265.- The orbits of Jupiter's moons are all in or near 
the plane of his equator ; and as his orbit nearly coin- 
cides with the ecliptic, and his equator with his orbit, it 
follows that, like our own moon, his satellites revolve 
near the plane of the ecliptic. On this account, they 
are sometimes between us and the planet, and sometimes 
beyond him, and seem to oscillate, like a pendulum, from 
their greatest elongation on one side to their greatest 
elongation on the other. 

266. Their direction is from west to east, or in the 
direction their primary revolves, both upon his axis and 
in his orbit. From the fact that their elongations east 
and west of Jupiter are nearly the same at every revolu- 
tion, it is concluded that their orbits are but slightly 
elliptical. They are supposed to revolve on their re- 
spective axes, like our own satellite, the moon, once 
during every periodic revolution. 

267. As these orbits lie near the plane of the ecliptic, 
they have to pass through his broad shadow when in 
opposition to the sun, and be totally eclipsed at every 
revolution. To this there is but one exception. As the 
fourth satellite departs about 3° from the plane of Jupi- 
ter's orbit, and is quite distant, it sometimes passes above 
or below the shadow, and escapes eclipse. But such 
escapes are not frequent. 

265. How are their orbits situated ? How satellites appear to move ? 

266. Direction of secondaries ? Form of orbits ? How ascertained ? 
What motion on axes ? 

267. What said of eclipses? Of fourth satellite ? Of solar eclipses upon 
Jupiter? Number of solar and lunar \ 

n* 



126 



ASTRONOMY. 



These moons are not only often eclipsed, but they often 
eclipse Jupiter, by throwing their own dark shadows 
upon his disk. They may be seen like dark round spots 
traversing it from side to side, causing, whenever that 
shadow falls, an eclipse of the sun. Altogether, about 
forty of these eclipses occur in the system of Jupiter 
every month. 

268. The immersions and emersions of Jupiter's moons 
have reference to the phenomena of their being eclipsed. 
Their entrance into the shadow is the immersion ; and 
their coming out of it the emersion, 

ECLIPSES OP JUPITER'S MOONS, EMERSIONS, ETC. 

//■ A 



\ 
\ B > 




1. The above is a perpendicular view of the orbits of Jupiter's satellites. His broad 
shadow is projected in a direction opposite the sun. At C, the second satellite is suffer- 
ing an immersion, and will soon be totally eclipsed: while at D, the first is in the act of 
emersion, and will soon appear with its wonted brightness. The other satellites are 
seen to cast their shadows off into space, and are ready in turn to eclipse the sun, or cut off 
a portion of his beams from the face of the primary. 

2. If the earth were at A in the cut. the immersion, represented at C. would be in- 
visible ; and if at B, the emersion at D could not be seen. So, also, if the earth were 
exactly at F, neither could be seen : as Jupiter and all his attendants would be directly 
beyond the sun, and would be hid from our view. 

269. The system of Jupiter may be regarded as a 
miniature representation of the solar system, and as fur- 
nishing triumphant evidence of the truth of the Coper- 
nican theory. It may also be regarded as a great natu- 
ral clock, keeping absolute time for the whole world ; as 
the immersions and emersions of his satellites furnish a 
uniform standard, and, like a vast chronometer hung up 
in the heavens, enable the mariner to determine his lon- 
gitude upon the trackless deep. 

268. What are the immersions and emersions of Jupiter's moons ? (Are 
the immersions and emersions always visible from the earth ? Why not ? 
Illustrate.) 

269. How may the system of Jupiter be regarded ? What use made of in 
navigation ? (Illustrate method. Much used ?) 



SATELLITES OF THE EXTERIOR PLANETS. 127 



By Ions and careful observations upon these satellites, astronomers have been able 
to construct tables, showing the exact time when each immersion and emersion will take 
place, at Greenwich Observatory, near London. Now suppose the tables fixed the time 
for a certain satellite to be eclipsed at 12 o'clock at Greenwich, but we find it to occur at 
9 o'clock, for instance, by our local time : this would show that our time was three hours 
behind the time at Greenwich; or, in other words, that we were three hours, or 45°, 
icest of Greenwich. If our time was ahead of Greenwich time, it would show that we 
were east of that meridian, to the amount of 15° for every hour of variation. But this 
method of finding the longitude is less used than the " lunar method" (Art. 245), on ac- 
count of the greater difficulty of making the necessary observations. 

270. By observations upon the eclipses of Jupiter's 
moons, as' compared with the tables fixing the time of 
their occurrence, it was discovered that light had a pro- 
gressive motion, at the rate of about 200,000 miles per 
second.. 

1. This discovery may be illustrated by again referring to the opposite cut. In the 
vear 1075, it was observed by Eoeiner, a Danish astronomer, that when the earth was 
nearest to Jupiter, as at E, the eclipses of his satellites took place 8 minutes 13 seconds 
sooner than the mean time of the tables ; but when the earth was farthest from Jupiter, 
as at F, the eclipses took place S minutes and 13 seconds later than the tables predicted, 
the entire difference being 16 minutes and 26 seconds. This difference of time ho 
ascribed to the progressive motion of light, which he concluded required 16 minutes and 
26 seconds to crossthe earth's orbit from E to F. 

2. This progress may be demonstrated as follows :— 1 6m. 26s. = 9S6s. If the radius of 
the earth's orbit be 9o millions of miles, the diameter must be twice that, or 190 mil- 
lions. Divide 190,000,000 miles by 9S6 seconds, and we have 192,697 J ^ miles as the 
progress of lisht in each second. At this rate, light would pass nearly eight times 
around the globe at every tick of the clock, or nearly 500 times every minute. 1 

SATURN. 

271. The moons of Saturn are eight in number, and 
are seen only with telescopes of considerable power. 
The best time for observ- 
ing them is when the 
planet is at his equinoxes, 
and his rings are nearly 
invisible. 

In January, 1S49, the author saw five 
of these satellites, as represented in the adjoining cut The rings appeared only as a 
line of light, extending each way from the planet and the satellites were in the direction 
of the line, at different distances, as here represented. 

272. These satellites all revolve eastward with the 
rings of the planet, in orbits nearly circular, and, with 
the exception of the eighth, in the plane of the rings. 
Their mean distances, respectively, from the planet's cen- 

270. What discovery by observing these eclipses? (Illustrate method. 
Diagram. Demonstration.) 

271. Number of Saturn's moons ? How seen ? Best time ? 

272. How revolve? Shape of orbits? How situated? Distances? 
Periods ? 



SATELLITES OF SATT7EN. 

fhpm is whfvn t.ViA . 




128 ASTRONOMY 



ter are from 123,000 to 2,366,000 miles ; and their pe- 
riods from 22 hours to 79 days, according to their dis- 
tances. 

The distances and periods of the satellites of Saturn are as follows: 

Distance in miles. Perioilic times. Distance in miles. Periodic times 

1st 123,000 day 22 hours. 5th 351,000 4 days 12 hours. 

2d 15S,000 1 " 8 " 6th 811,000 15 u 22 u 

3d 196,000 1 " 21 " 7th .... 2,366,000 79 " 7 " 

4th 251,000 2 " 17 " 

COMPARATIVE DISTANCES OF TIIE MOONS OF SATURN. 
12 3*6 6 7 « 



SYSTEM OF SATURN — NO ECLIPSES. 



273. The most distant of these satellites is the largest, 
supposed to be about the size of Mars ; and the remain- 
der grow smaller as they are nearer the primary. They 
are seldom eclipsed, on account of the great inclination 
of their orbits to the ecliptic, except twice in thirty years, 
when the rings are edgewise toward the sun. The eighth 
satellite, which has been studied more than all the rest, 
is known to revolve once upon its axis during every 
periodic revolution ; from w T hich it is inferred that they 
all revolve on their respective axes in the same manner. 

1. Let the line A B represent the 
plane of the planet's orbit, C D his 
axis, and EF the 'plane of his rings. 
The satellites being in the plane of the 
rings, will revolve around the shadow 
of the primary, instead of passing 
through it, and being eclipsed. 

2. At the time of his equinoxes, how- 
ever, when the rings are turned toward 
the sun (see A and E, cut, page 92), 
they must be in the center of the shad- 
ow on the opposite side ; and the 

moons, revolving in the plane of the rings, must pass through the shadow at every 
revolution. The eighth, however, may sometimes escape, on account of his departure 
from the plane of the rings, as shown in the cut. 

URANUS. 

274. Uranus is supposed to be attended by six secon- 
daries. Sir Win. Herschel recorded that he saw this 
number, and computed their periods and distances ; and 
on his authority the opinion is generally received, though 

273. Size ? Eclipses of? When ? Why not oftener ? (Illustrate.) 

274. Satellites of Uranus ? Upon what authority ? Distances ? Periods ? 
Situation of orbits? Form? Direction in revolution? Remark of Dr. 
Herschel ? 




NATURE AND CAUSE OF TIDES. 129 



no other observer has ever been able to discover more 
than three. They are situated at various distances, and 
revolve in from 1 day and 21 hours to 117 days. Their 
orbits are nearly perpendicular to the ecliptic, and they 
revolve backward, or from east to west, contrary to all 
the .other motions of our planetary system. Their or- 
bits are nearly circular, and they are described by Dr. 
Herschel as " the most difficult objects to obtain a sight 
of, of any in our system." 

The distances and periods of the system of Uranus, as laid down by Dr. Herschel, are 
as follows : ' 

Distance in miles. Periodic times. Distance in miles. Periodic times. 

1st 224,000 5days 21 hours. 4th 390,000 13 days 11 hours. 

2d 206,000 8 " 17 " 5th 777,000 3S " 2 " 

3d 340,000 10 " 23 " | 6th .... 1,556,000 117 " 17 " 

NEPTUNE. 

275. Neptune is known to be attended by one satel- 
lite, and suspected of having two. Professor Bond, of 
Cambridge, Mass., states that he has at times been quite 
confident of seeing a second. The mean distance of the 
known satellite from its primary is 230,000 miles, or near 
the distance of our moon. Its period is only 5 days and 
21 hours. 

We have here another illustration of the great law of planetary motion explained at 
74. So great is the attractive power of Neptune, that to keep a satellite, at the distance 
of our moon, from falling to his surface, it must revolve some five times as swiftly as she 
revolves around the earth. The centripetal and centrifugal forces must be balanced in 
all cases, as the laws of gravitation and planetary motion, discovered by Newton and 
Kepler, extend to and prevail among all the secondaries. 



CHAPTER VIII. 

NATURE AND CAUSE OF TIDES. 

276. Tides are the alternate rising and falling of the 
waters of the ocean, at regular intervals. Flood tide is 
when the waters are rising / and ebb tide, when they are 

275. What said of Neptune's secondaries? Kemark of Prof. Bond ? Dis- 
tance and period of the Known satellite ? (Remark in note.) 

276. What are tides ? Flood and ebb tides ? High and low? How often 
do they ebb and now ? 

6* 



130 ASTRONOMY. 



NO TIDE. 



falling. The highest and lowest points to which they 
go are called, respectively, high and low tides. The 
tides ebb and flow twice every twenty-four hours — i. e., 
we have two flood and two ebb tides in that time. 

277. The tides are not uniform, either as to time or 
amount. They occur about 50 minutes later every day 
(as we shall explain hereafter), and sometimes rise much 
higher and sink much lower than the average. These 
extraordinary high and low tides are called, respectively, 
spring and neap tides. 

278. The cause of the tides is the attraction of the sun 
and moon upon the waters of the ocean. But for this 
foreign influence, as w T e may call it, the waters having 
found their proper level, w^ould cease to heave and swell, 
as they now do, from ocean to ocean, and 
would remain calm and undisturbed, save 
by its own inhabitants and the winds of 
heaven, from age to age. 

In this figure, the earth is represented as surrounded by water, in a 
state of rest or equilibrium, as it would be were it not acted upon by 
the sun and moon. 

279. To most minds, it would seem that the natural 
effect of the moon's attraction w r ould be to produce a 
single tide-wave on the side of the earth toward the 
moon. It is easy, therefore, for students to conceive how 
the moon can produce one flood and one ebb 
tide in twenty-four hours. 

1. In this cut, the moon is shown at a distance above the earth, 
and attracting the waters of the ocean, so as to produce a high tide 
at A. But as the moon makes her apparent westward revolution 
around the earth but once a day, the simple raising of a flood tide 
on the side of the earth toward the moon, would give us but one flood 
and one ebb tide in twenty-four hours ; whereas it is known that we 
have two of each. 

2. " The tides, 1 ' says Dr. Herschel, " are a subject on which many 
persons find a strange difficulty of conception. That the moon, by her 
attraction, should heap up the waters of the ocean under her, seems 
to many persons very natural. That the same cause should, at the 
same time, heap them up on the opposite side of the earth (viz., at B in the figure), seems 
to many palpably absurd. Yet nothing is more true." 

280. Instead of a single tide-wave upon the waters of 

277. Are the tides uniform ? What variation of time ? As to amount ? 
W T hat are these extraordinary high and low tides called ? 

278. The cause of tides ? How but for this influence ? 

279. What most obvious effect of the moon's attraction ? (Substance of 
note 1 ? Kemark of Dr. Herschel ?) 




ONE TIDE-WAVE. 




NATURE AND CAUSE OF TIDES. 131 




the globe, directly under the moon, it is found that on 
the side of the earth directly opposite there is another 
high tide ; and that half way between these two high 
tides are two low tides. These four tides, TW0 TIDE . WAVEflh 
viz., two high and two low, traverse the ^ 

ocean from east to west every day, which flJ A 

accounts for both a flood and an ebb tide 
every twelve hours. 

In this cut, we have a representation of the tide-waves as they 
actually exist, except that their night, as compared with the magni- 
tude of the earth, is vastly too great. It is designedly exaggerated, 
the better to illustrate the principle under consideration? While 
the moon at A attracts the waters of the ocean, and produces a high 
tide at B, we see another high tide at C on the opposite side of the 
globe. At the same time it is low tide at D and E. 

281. The principal cause of the tide-wave on the side 
of the earth opposite the moon is the difference of the 
moon's attraction on different sides of the earth. 

If the student well understands the subject of gravitation (65), he will easily perceive 
how a difference of attraction, as above described, would tend to produce an elongation 
of the huge drop of water called the earth. The diameter of the earth amounts to about 
55 th of the moon's distance : so that, by the rule (69), the difference in-her attraction 
on the side of the earth toward her, and the opposite side, would be about y\th. The 
attraction being stronger at B (in the last cut) than at the earth's center, and stronger at 
her center than at C, would tend to separate these three portions of the globe, giving 
the waters an elongated form, and producing two opposite tide-waves, as shown in the 
cut. 

282. A secondary cause of the tide-wave on the side 
of the earth opposite the moon, is the revolution of the 
earth around the common center of gravity between the 
earth and moon, thereby generating an increased centri- 
fugal force on that side of the earth. 

The center of gravity between the earth and moon is the point where they would 
exactly balance each other, if connected by a rod, and poised upon a fulcrum. 

CENTER OF GRAVITY BETWEEN THE EARTH AND MOON. 

Moon 




This point, which, according to Ferguson, is about 6,000 miles from the earth's center, 
is represented at A in the above, and also in the next cut. 

280. How many tide-waves are there on the globe, and how situated ? 

281. State the principal cause of the wave opposite the moon? (Demon- 
strate by diagram.) 

282. What other cause operates with the one just stated to produce the 
tide-wave opposite the moon ? (What is the center of gravity between the 
earth and the moon ? Where is it situated ? Illustrate the operation of this 
secondary cause. Diagram.) 



132 



ASTRONOMY. 



SECONDARY CAUSE OF HIGH TIDE OPPOSITE THE MOON. 




TIDE-WAYES BEHIND THE MOON. 



1. The point A represents the center of gravity between the ear h and moon ; and as 
it is this point which traces the regular curve of the earth's orbit, it is represented in 
the arc of that orbit, while the earth's center is 6,000 miles one side of it. Now the law 
of gravitation requires that while both the moon and earth revolve around the sun, they 
should also revolve around the common center of gravity between them, or around the 
point A. This would give the earth a third revoUvtion, in addition to that around the 
sun and on her axis. The small circles show her path around the center of gravity, 
and the arrows her direction. 

2. This motion of the earth would slightly increase the centrifugal tendency at B, 
and thus help to raise the tide-wave opposite the moon. But as this motion is slow, 
corresponding with the revolution of the moon around the earth, the centrifugal force 
could not be greatly augmented by such a cause. 

283. As the moon, which is the principal cause of the 
tides, is revolving eastward, and comes to the meridian 
later and later every night, so the tides are about 50 
minutes later each successive day. This makes the in- 
terval between two successive high tides 12 hours and 25 
minutes. Besides this daily lagging 

with the moon, the highest point 
of the tide-wave is found to be 
about 45° behind or east of the 
moon, so that high tide does not 
occur till about three hours after 
the moon has crossed the merid- 
ian. The waters do not at once 
yield to the impulse of the moon's 
attraction, but continue to rise 
after she has passed over. 

In the cut, the moon is on the meridian, but the highest point of the wave is at A, or 
45° east of the meridian ; and the corresponding wave on the opposite side at B is 
equally behind. 

284. The time and character of the tides are also 
affected by winds, and by the situation of different places. 
Strong winds may either retard or hasten the tides, or 
may increase or diminish their hight ; and if a place is 
situated on a large bay, with but a narrow opening into 

be longer in rising, as the bay has 




the sea, the tide will 



283. What daily lagging of the tides ? Interval between two successive 
high tides ? What other lagging ? Cause of this last ? 

284. What modification of the time and character of the tides ? 



NATUKE AND CAUSE OF TIDES. 133 



to fill up through a narrow gate. Hence it is not usually 
high tide at New York till eight or nine hours after the 
moon has passed the meridian. 

285. As both the sun and moon are concerned in the 
production of tides, and yet are constantly changing 
their positions with respect to the earth and to each 
other, it follows that they sometimes act against each 
other, and measurably neutralize each other's influence ; 
while at other times they combine their forces, and mutu- 
ally assist each other. In the latter case, an unusually 
high tide occurs, called the Spring Tide. This happens 
both at new and full moon. 

CAUSE OF SPRING TIDES. 







%i^jj 



^4A.^ V 



1. Here the sun and moon, being in conjunction, unite their forces to produce an ex- 
traordinary tide. The same effect follows when they are in opposition ; so that we have 
two spring tides every month — namely, at new and full moon. 

2. If the tide-waves at A and B are one-third higher at the moon's quadrature than 
usual, those of C and D will be one-third loicer than usual. 

286. Although the sun attracts the earth much more 
powerfully, as a whole, than the moon does, still the 
moon contributes more than the sun to the production 
of tides. Their relative influence is as one to three. 
The nearness of the moon makes the difference of her 
attraction on different sides of the earth much greater 
than the difference of the sun's attraction on different 
sides. 

It must not be forgotten that the tides are the result not so much of the attraction of 
the sun and moon, as a whole, as of the difference in their attraction on different sides 

285. Do the sun and moon always act together in attracting the waters ? 
"Why not? How affect each other's influence ? Effect on the tides ? What 
are Spring Tides ? When do they occur ? (Illustrate by diagram the cause 
of spring tide, when the sun and moon are in conjunction.) 

286. Comparative influence of sun and moon in*tlie production of tides? 
Why moon's influence the greatest? (Substance of note ? Demonstration.) 

12 



134 



ASTRONOMY. 




$>&&, 



'j&\ r 'i; 




<%&& 







of the earth, caused by a difference in the distances of the several parts. The attrac- 
tion being inversely, as the square of the distance (69), the influence of the sun and 
moon, respectively, must be in the ratio of the earth's diameter to their distances. Now 
the difference in the distance of two sides of the earth from the moon is ^th of the 
moon's distance ; as 240,000 -r- 8,000 = 30 ; while the difference, as compared with the 
distance of the sun, is only TT * T? th, as 95,000,000 -j- 8,000 = 11,875. 

287. When the moon is in quadrature, and her influ- 
ence is partly neutralized by the sun, which now acts 
against her, the result is a very low tide, called Neap 

Tide 

SPRING AND NEAP TIDE£. 

The whole philosophy of spring ..•—•_—.. 

and neap tides may be illustrated by 
the annexed diagram. 

1. On the right side of the cut, the 
sun and moon are in conjunction, 
and unite to produce a spring tide. 

2. At the first quarter, their at- 
traction acts at right angles, and the 
sun, instead of contributing to the 
lunar tide-waves, detracts from it to 
the amount of his own attractive 
force. The tendency to form a tide 
of his own, as represented in the 
figure, reduces the moon's wave to 
the amount of one-third. 

3. At the full moon, she is in oppo- 
sition to the sun, and their joint at- 
traction acting again in the same 
line, tends to elongate the fluid por- 
tion of the earth, and a second spring 
tide is produced. 

4. Finally, at the third quarter, 
the sun and moon act against each 

other again, and the second neap tide is the result. Thus we have two spring and two 
neap tides during every lunation — the former at the moon's syzygies, and the latter at her 
quadratures. 

288. The tides are subject to another periodic varia- 
tion, caused by the declination of the sun and moon 
north and south of the equator. As tides affected by declina- 
the tendency of the tide-wave is to TI0N * 

rise directly under the sun and ..$)' 4D... 

moon, when they are in the south, / 
as in winter, or in the north, as in \ i 

summer, every alternate tide is \ ; / 

higher than the intermediate one. 



«>/N(* V > 



is P^\ 




-*M?X&/ 




At the time of the equinoxes, the sun being over the 
equator, and the moon within 5^° of it, the crest of the 
great tide-wave will be on the equator ; but as the sun 
and moon decline south to A, one tide-wave forms in 
the south, as at B, and the opposite one in the north, as at C. If the declination was 
north, as shown at D, the order of the tides would be reversed. The following diagram, 

287. What are Neap Tides f Their cause ? (Illustrate entire philosophy 
by diagram.) 

288. "What other periodic variations mentioned? (Explain cause, and 
illustrate.) 



NATURE AND CAUSE OF TIDES. 



135 



if carefully studied, will more fully illustrate the subject of the alternate high and low 
tides, m high latitudes, in winter and summer: 

ALTERNATE HIGH AND LOW TIDES. 

N ^ B 

DEC 23 




1. Let the line A A represent the plane of the ecliptic, and B B the equinoctial. On 
the 21st of June, the day tide-wave is north, and the evening wave south, so that the 
tide following about three hours after the sun and moon will "be higher than the inter- 
mediate one at 3 o'clock in the morning. 

2. On the 23d of December, the sun and moon being over the southern tropic, the 
highest wave in the southern hemisphere will be about3 o'clock P. M., and the lowest 
about 8 o'clock A.M.; while at the north, this order will be reversed. It is on this ac- 
count that in high latitudes every alternate tide is higher than the intermediate ones ; 
the evening tides in summer exceeding the morning tides, and the morning tides in win- 
ter exceeding those of evening. 

289. All spring and neap tides are not alike as to their 
elevation and depression, As the distances of the sun 
and moon are varied, so are the tides varied, especially 
by the variations of the moon. 

VARIATIONS IN THE SPRING TIDES. 




G. 



^s 



t 







1. At A, the earth is in aphelion, and the moon in apogee. As both the sun and moon 
are at their greatest distances, the earth is least affected by their attraction, and the spring 
tides are proportionately low. 

2. At B, the earth is m perihelion, and the moon in perigee; so that both the sun and 
moon exert their greatest influence upon our globe, and the spring tides are highest, as 
shown in the figure. In both cases, the sun and moon are in conjunction, but the varia- 
tion in the distances of the sun and moon causes variations in the spring tides. 

290. In the open ocean, especially the Pacific, the tide 
rises and falls but a few feet ; but when pressed into nar- 
row bays or channels, it rises much higher than under 
ordinary circumstances. 



289. Are all spring and neap tides alike ? By what are they modified ? 
(Illustrate by diagram.) 

290. Hight of tides in open seas ? How in narrow bays and channels ? 
(Hight at different points on our coast?) 



ASTRONOMT, 



The average elevation of the tnle I points on our coast is as follows: 

Cumberland, head of the Bay of Fundy 71 feet. 

Boston ". * 11 

New Haven S " 

New York 5 " 

Charleston, S. C 6 " 

291, As the great tide-waves proceed from east to 
west, they are arrested by the continents, so that the 
waters are permanently higher on their east than on their 
west sides. The Gulf of Mexico is 20 feet higher than 
the Pacific Ocean, on the other side of the Isthmus ; and 
the Red Sea is 30 feet higher than the Mediterranean. 
Inland seas and lakes have no perceptible tides, because 
they are too small, compared with the whole surface of 
the globe, to be sensiblv affected bv the attraction of the 
snn and moon. 

We have thus stated the principal facts connected with 
this complicated phenomenon, and the causes to which 
they are generally attributed. And vet it is not certain 
that the philosophy of tides is to this day fully under- 
stood. La Place, the great French mathematician and 
astronomer, pronounced it one of the most difficult prob- 
lems in the whole range of celestial mechanics. It is 
probable that the atmosphere of our globe has its tides, 
as well as the waters ; but we have no means, as yet, for 
definitely ascertaining the fact. 



CHAPTER IX. 



OF COMETS. 



j 



292. Comets are a singular class of bodies, belonging 
to the solar system, distinguished for their long trains of 
light, their various shapes, and the great eccentricity of 
their orbits. Their name is from the Greek coma, which 



291. Direction of tide-waves ! What result \ Instances cited ? Have in- 
land seas and lakes any tides \ Why not \ Remarks respecting philosophy 
of tides ? Of La Place ? Atmospheric tides ! 

292. What are comets ? Derivation of name ? Are they opake or self- 
luminous ? 



OF COMETS. 



1° 



) » 



GREAT COMET OF 371 BEFORE CUEIST. 



signifies heard or hair, on account of their bearded or 
hairy appearance. They are known to be opake, from 
the fact that they sometimes exhibit phases, which show 
that they shine only by reflection. 

293. Comets usually consist of three parts — the nu- 
cleus, the envelope, and the tail. The nucleus is what 
may be called the lody or head of the comet. The 

lope is the nebulous or hairy covering that sur- 
rounds the nucleus ; and 
the tail is the expan- 
sion or elongation of the 
envelope. But all comets 
have not these parts. 
Some have no percept- 
ible nucleus ; their entire 
structure being like that 
of a thin vapory cloud 
passing through the dis- 
tant heavens. Others 
have but a slight envelope 
around a strongly marked 
nucleus. 

The great comet that appeared 371 
years before Christ exhibited the different parts of a comet -with great distinctness: en 
which account, as well as for its striking magnificence, we give a view of it in the above 

294. The tails of comets usually lie in a direction 
opposite to the sun, so that from perihelion to aphelion 
they precede their nuclei or heads ; or, in other words, 
comets seem, after having passed their perihelion, to JklcU 
out of the solar system. Their tails are usually curced 
more or less, being concave toward the region from 
whence they come. This is well shown in the comets of 
1811, 1843, and in the following cut. That of 1689 is 
said to have been curved like a Turkish sabre. The 
cause of this curvature of the tails of comets is supposed 
to be a very rare ethereal substance which pervades 




293. Parts of a comet \ Describe each. Have all comets these three parts 



(What comet shown as a sample in the c 

204. Direction of the tails of comets ? How curved ? 
vat lire ? 



so of this cur- 



138 



ASTRONOMY 



space, and offers a slight resistance to their progress. Of 
course it must be almost infinitely attenuated, as the 
comets themselves are a mere vapor, which could make 
no progress through the spaces of the heavens, were they 
not very nearly a vacuum. They could no more pass a 
medium as dense as our atmosphere, than an ordinary 
cloud could pass through the waters of the sea. 

295. The form of the comets' orbits is generally that 
of an ellipse greatly flattened or elongated. The sun 
being near one end of the ellipse, and the planets com- 
paratively in his immediate neighborhood, the comets 
are in the vicinity of the sun and planets but a short 
time, and then hasten outward again beyond the limits of 
human vision, with the aid of the best telescopes, to be 
gone again for centuries. 

ORBIT OF A COMET. 




Here it will be seen that the orbit is very eccentric, that the perihelion point is very 
near the sun, and the aphelion point very remote. 

296. The tails of comets do not continue of the same 
uniform lengtli. They increase both in length and 
breadth as they approach the sun, and contract as they 
recede from him, until they often nearly disappear before 
the comet gets out of sight. Instances have occurred in 
which tails of comets have been suddenly expanded or 
elongated to a great distance. This is said to have been 
the case with the great comet of 1811. 

295. Form of the orbits of comets ? What near the earth but little of the 
time? 

296. What said of the contraction and expansion of the tails of comets ? 
What specimen shown in the cut ? 



OF COMETS. 



139 



GREAT COMET OF 1811. 






COMET OF 1744. 



297. Comets have been known to exhibit several tails 
at the same time. That of 1744, represented in the cut, 
had no less than six tails 

spread out in the heavens, 
like an enormous fan. The 
comet of 1823 is said to have 
had two tails, one of which 
extended toward the sun. 

The comet of 1T44, represented in this cut, 
excited great attention and interest. It ex- 
hibited no train till within the distance of the 
orbit of Mars from the sun ; but early in 
March it appeared with a tail divided into six 
branches, all diverging, but curved in the 
same direction. Each of these tails was about 
40 wide, and from 30° to 44° in length. The 
edges were bright and decided, the middle 
faint, and the intervening spaces as dark as 
the rest of the firmament, the stars shining 
in them. When circumstances were favor- 
able to the display of this remarkable body, 
the scene was striking and magnificent, al- 
most beyond description. 

298. The heads or nuclei of comets are comparatively 
small 
eter in five different instances 




The following table shows the estimated diam- 



The comet of 1778 
" 1805 

" 1799 

" 1807 

" 1811 



diameter of head 33 miles. 

U it QQ U 

" " 462 " 

a u 666 tt 

" " 428 " 



297. Have they ever more than one tail ? What peculiarity of the comet 
of 1823 ? (What specimen of comet with several tails ? Describe.) 



140 



ASTRONOMY. 



COMET OF 1585. 



Many comets have simply the envelope, without any tail 
or elongation. Such were those that appeared in 1585 
and 1763, the former of which is 
represented in the adjoining cut. 
Cassini describes the comet of 
1682 as being as round and as 
bright as Jupiter, without even an 
envelope. But these are very rare 
exceptions to the general charac- 
ter of cometary bodies. 

299. The tails of comets are 
often of enormous length and 
magnitude. That of 371 before 
Christ was 60° long, covering one-third of the visible 
heavens. In 1618, a comet appeared, which was 101° 
in length. Its tail had not all risen when its head 
reached the middle of the heavens. That of 1680 had 
a tail 70° long ; so that though its head set soon after 
sundown, its tail continued visible all night. 

GREAT COMET OF 1843. 





The following table will show the length of the tails of some of the most remarkable 
comets, both in degrees and in miles. They will be characterized only by the year when 
they appeared : 



B 


C. 


371 


A 


D.1456 




u 


161S 




H 


16S0 




(( 


1689 



Desr. Miles. 

60 140,000,000 

60 70,000,000 

104 65,000,000 

70 123.000,000 

03 100,000,000 



Deg. Miles. 

A. D. 1744 30 35.000,006 

" 1769 90 43.000,000 

" 1S11 23 132,000,000 

" 1843 60 130,000,000 



29S. W r hat of the size of the nuclei of comets ? Give a few examples. 
What comets without tails ? What specimen in the cut? What said of the 
comet of 1682 ? Are such comets numerous ? 

299. W T hat of the size of the tails of comets? That of 371 B. C. ? Of 
A. D. 1618 ? Of 1680 ? (What specimen in cut, and its length ? State the 
length of some others in miles.) 



OF COMETS. 



141 



300. The velocity with which comets often move is 
truly wonderful. Their motions are accelerated as they 
approach, and retarded as they recede from the sun ; so 
that their velocity is greatest while passing their peri- 
helions. The comet of 1472 described an arc of the 
heavens of 120° in extent in a single day ! That of 
1680 moved, when near its perihelion, at the rate of 
1,000,000 miles per hour. 

301. The temperature of some comets, when nearest 
the sun, must be very great. That of 1680 came within 
130,000 miles of the sun's surface, and must have re- 
ceived 28,000 times the light and heat which the earth 
receives from the sun — a heat more than 2,000 times 
greater -than that of red-hot iron ! What substance can 
a comet be composed of to endure the extremes of heat 
and cold to which it is subject? Some have supposed 
that their tails were caused by the sun's light and heat 
rarefying and driving back the vapory substance com- 
posing the envelope. 

302. The periods of but few comets are known. That of 
1818, called EncMs Co- 
met, has a period of only 
3^- years. BielcCs Comet 
has a period of 6| years. 
That of 1682 (then first 
noticed with care, and 
identified as the same 
that had appeared in 
1456, 1531, and 1607) 
has a period of about 76 
years. It is called Hal- 
leifs Comet, after Dr. 
Halley, who determined 
its periodic time. The 
great comet of 1680 has a periodic time of 570 years, 
so that its next return to our system will be in the year 



ENCKE S COMET. 




800. Velocity of comets ? Uniform or not ? Comet of 1472 ? Of 16S0 ? 

301. Temperature? Comet of 1680 ? Supposed cause of their tails ? 

302. Periods? Encke's ? Biela*s ? Bailey's? That of 1680? Supposed 
periods of others? Opinions of Prof. Nichol and Dr. Herschel ? 



142 ASTRONOMY. 



2250. Many are supposed to have periods of thousands 
of years ; and some have their orbits so modified by the 
attraction of the planets, as to pass off in parabolic curves, 
to return to our system no more. 

Prof. Nichol is of opinion that the greater number visit our system but once, and then 
fly off in nearly straight lines till they pass the center of attraction between the solar 
system and the fixed stars, and go to revolve around other suns in the far distant heav- 
ens. Sir John Herschel expresses the same sentiment. 

303. The distances to which those comets that return 
must go, to be so long absent, must be very great. Still 
their bounds are set by the great law of gravitation, for 
were they to pass the point " where gravitation turns the 
other way," they w T ould never return. But some, at 
least, do return, after their " long travel of a thousand 
years." What a sublime conception this affords us of 
the almost infinite space between the solar system and 
the fixed stars. 

ORBIT OF HALLEY'S COMET. 




304. The perihelion distances of the various comets 
that have appeared, and whose elements have been esti- 
mated by astronomers, are also exceedingly variable. 
"While some pass very near the sun, others are at an im- 
mense distance from him, even at their perihelion. Of 
137 that have been particularly noticed, 

30 passed between the sun and the orbit of Mercury. 
44 between the orbits of Mercury and Venus. 
34 " " Venus and the earth. 

23 " " the earth and Mars. 

6 " " Mars and Jupiter. 

308. Distances to which they go ? Remark respecting the law of gravi- 
tation ? What specimen of orbit given ? 
304. What said of perihelion distances ? < How many noticed ? Where did 






OF COMETS. 143 






The orbit of Encke's comet is wholly within the orbit of Jupiter, while that of 
Biela's extends but a short distance beyond it. The aphelion distance of Halley s comet 
is 3,400 millions of miles, or 550 millions 

of miles beyond the orbit of Neptune. orbits of several comets. 

But these are all comets of short periods. 

305. The number of „. • * 

comets belonging to, or \*> ••.. ^ 

that visit the solar system, / encke's a 

is very great. Some have £ bailed ? ff Vi? ..• - : " * "" "^^'%\ 

estimated them at several ""] '"iU-^-^f^ J \ 

millions. "When we con- | f\Q% "^- -'' \ 

sider that most comets are I [VS :£ -"'" \ I 

seen only through tele- \ 
scopes — an instrument of 
comparatively modern \ /?i 

date — and that, notwith- x x -.F 

standing this, some 450 ,'"' ""■ :"""' 

are mentioned in ancient 

annals and chronicles, as having been seen with the 
naked eye, it is probable that the above opinion is by no 
means extravagant. It is supposed that not less than 700 
have been seen at different times since the birth of 
Christ. The paths of only about 140 have been deter- 
mined. 

The extreme difficulty of observing comets whose nearest point is beyond the orbit 
of Mars, is supposed to account for the comparatively small number that have been seen 
without that limit; and the proximate uniformity of the distribution of their orbits 
over the space included within the orbit of Mars, seems to justify the conclusion, that 
though seldom detected beyond his path, they are nevertheless equally distributed 
through all the spaces of the solar heavens. Reasoning upon this hypothesis, Professor 
Arago concludes that there are probably seven millions of comets that belong to or 
visit the solar system. 

306. The directions of comets are as variable as their 
forms or magnitudes. They enter the solar system from 
all points of the heavens. Some seem to come up from 
the immeasurable depths below the ecliptic, and, having 
doubled " heaven's mighty cape," again plunge down- 
ward with their fiery trains, and are lost for ages in the 
ethereal void. Others appear to come down from the 
zenith of the universe, and, having passed their peri- 

thev pass ? (What samples given in cut ? Where does the orbit of Encke's 
comet lie ? Of Biela's ? Of HaileVs ?) 

005. The number of comets? What estimate? Why probably correct? 
How many supposed to have been seen since the birth of Christ f (Why so 
few seen ? How supposed to be distributed ? What conclusion of Arago ?) 

306. Direction of comets ? (Remark of late writer ?) 



144 ASTRONOMY. 



helion, reascend far above all human vision. Others 
again are dashing through the solar system, in all possible 
directions, apparently without any prescribed path, or any 
guide to direct them in their eccentric wanderings. In- 
stead of revolving uniformly from east to west, like the 
planets, their motions are direct, retrograde, and in every 
conceivable direction. 

It is remarked by a late writer, that the average inclinations of all the planes in 
which the comets now on record have been found to move, is about 90°. This he re- 
gards as a wonderful instance of the goodness of Providence, in causing their motions 
to be performed in a manner least likely to come in contact with the earth and the other 
planets. 

307. Of the physical ncdure of comets, little is known. 
That they are, in general, very light and vapory bodies, 
is evident from the fact that stars have sometimes been 
seen even through their densest portions, and are gene- 
rally visible through their tails, and from the little attrac- 
tive influence they exert upon the planets in causing 
perturbations. "While Jupiter and Saturn often retard 
and delay comets for months in their periodic revolutions, 
comets have not power, in turn, to hasten the time of the 
planets for a single hour ; showing conclusively that the 
relative masses of the comets and planets are almost in- 
finitely disproportionate. 

Such is the extreme lightness or tenuity of cometary bodies, that in all probability 
the entire mass of the largest of them, if condensed to a solid substance, would not 
amount to more than a fewhundred pounds. Sir Isaac Newton was of opinion, that if 
the tail of the largest comet was compressed within the space of a cubic inch, it would 
not then be as dense as atmospheric air! The comet of 1770 got entangled, by attrac- 
tion, among the moons of Jupiter, on its way to the sun, and remained near them for 
four months ; yet it did not sensibly affect Jupiter or his moons. In this way the 
orbits of comets are often entirely changed. 

308. Comets were formerly regarded as harbingers of 
famine, pestilence, war, and other dire calamities. In 
one or two instances, they have excited serious appre- 
hension that the day of judgment was at hand, and that 
they were the appointed messengers of Divine wrath, 
hasting apace to burn up the world. A little reflection, 
however, w T ill show that all such fears are groundless. 
The same unerring hand that guides the ponderous planet 

307. Physical nature of comets ? What proofs of their light and vapory 
character'/ (What said of their probable mass? Opinion of Newton ? 
What said of the comet of 1770 ? What effect on orbits ?) 

303. How cornets formerly regarded ? Why no fears of collision? (What 
estimate of "chances?"^ 



OF COMETS. 145 



in its way, directs also the majestic comet ; and where 
infinite wisdom and almighty power direct, it is almost 
profane to talk of collision or accident. 

Even those who have calculated the " chances" of collision — as if chance had any 
thing to do among the sotor bodies — have concluded the chances of collision are about 
as one to 2S1, 000,000 — i. e., like the chance one would have in a lottery, where there 
were 2S1, 000,000 black balls, and but one white one ; and where the white ball must be 
produced at the first. drawing to secure a prize. 

309. Were a collision actually to take place between 
a comet and the earth, it is not probable that the former 
would even penetrate our atmosphere, much less dash 
the world to pieces. Prof. Olmsted is of opinion that 
in such an event, not a particle of the comet would reach 
the earth — that the portions encountered by her would 
be arrested by the atmosphere, and probably inflamed ; 
and that they would perhaps exhibit, on a more magnifi- 
cent scale than was ever before observed, the phenomena 
of shooting stars or meteoric showers. The idea, there- 
fore, that comets are dangerous visitants to our system, 
has more support from superstition than from reason or 
science. 

The air is to us what the waters are to fish. Some fish swim around in the Seep, 
while others, like lobsters and oysters, keep on the bottom. So birds wing the air, 
while men and beasts are the " lobsters' 1 that crawl around on the bottom. Now there 
is no more probability that a comet would pass through the atmosphere, and injure us 
upon the earth, than there is that a handful of fog or vapor thrown down upon the sur- 
face of the ocean, would pass through and kill the shell-fish at the bottom. 

310. After all that is supposed to be known respecting 
comets, it must be admitted that they are less under- 
stood than any other bodies belonging to our system. 
" What regions these bodies visit, when they pass beyond 
the limits of our view ; upon what errands they come, 
when they again revisit the central parts of our system ; 
what is the difference between their physical constitution 
and that of the sun and planets ; and what important 
ends they are destined to accomplish in the economy of 
the universe, are inquiries which naturally arise in the 
mind, but which surpass the limited powers of the human 
understanding at present to determine." 



300. What probable effect in case of collision? Prof. Olmsted's opinion ? 
(Remark respecting the air, fish, lobsters, &c. ?) 

310. Are we as well acquainted with comets as with other bodies of our 
system ? What inquiries suggested ? How answered ? 



146 



ASTRONOMY, 



CHAPTER X 



OF THE SUN. 



THE SUN AND THE MOON'S ORBIT. 



311. Of all the celestial objects with which we are 
acquainted, none make so strong and universal an im- 
pression upon our globe as does the sun. He is the great 
center of the solar system — a vast and fiery orb, kindled 
by the Almighty on the morn of creation, to cheer the 
dark abyss, and to pour his radiance upon surrounding 
worlds. Compared with him, all the solar bodies are of 
inconsiderable dimensions ; and without him, they would 
be wrapped in the gloom of interminable night. 

312. The form of the sun is that of an oblate sphe- 
roid, his equatorial being somewhat greater than his 
polar diameter. The mean of the two is 886,000 miles. 
He is 1,400,000 times as large as the mighty globe we 
inhabit, and 500 times as large as all the planets put 
together. Were he placed 
where the earth is, he would 
fill all the orbit of the moon, 
and extend 200,000 miles be- 
yond it in every direction. It 
would take 112 such worlds 
as ours, if laid side by side, to 
reach across his vast diameter. 

1. The vast magnitude of the sun may be 
Inferred from the fact, that when rising: or set- 
ting, he often appears larger than the largest 
building, or the tops of the largest trees. 1ST ow 
if the angle filled by him at the distance of two 
miles is over 100 feet across, what must it be 
at the distance of 95 millions of miles? 

2. Were a railroad passed through the sun's center, and should a train of*cars start 
from one side, and proceed on at the rate of 30 miles an hour, it would require 3^ years 

311. Describe the sun. How compare with the rest of the system ? 

312. What is his form? Diameter? Mass, as compared with our globe? 
With all other bodies of the system ? With moon's orbit ? (What sensible 
evidence of the vast magnitude of the sun ? Illustration from railroad ? 
Demonstration as to its comparison with moon's orbit ?) 




OF THE SUN. 



147 



SPOTS ON THE SUN. 



to cross over his diameter. To traverse his vast circumference, at the same rate of 
speed, would require nearly 11 years. 

8. The mean distance of the moon from the earth's center is 240,000 miles ; conse- 
quently the diameter of her orbit, which is twice the radius, is 4S0,O00. Subtract this 
from 886,000, the sun's diameter, and we have 406,000 miles left, or 203,000 miles on each 
side, beyond the moon's orbit. 

313. By the aid of telescopes, a variety of spots are 
often discovered upon the sun's disk. Their number is 
exceedingly variable at different times. From 1611 to 
1629, a period of 18 years, the sun was never found clear 
of spots, except for a few 
days in December, 1624. 
At other- times, twenty or 
thirty were frequently seen 
at once ; and at one period, 
in 1825, upwards of fifty 
were to be seen. Prof. 
Olmsted states that over 
100 are sometimes visible. 
From 1650 to 1670, a pe- 
riod of 20 years, scarcely 
any spots were visible ; and 
for eight years, from 1676 
to 1684, no spots whatever 
were to be seen. For the last 46 years, a greater or less 
number of spots have been visible every year. For 
several days, during the latter part of September, 1846, 
we could count sixteen of these spots, which were dis- 
tinctly visible, and most of them well defined ; but on 
the 7th of October following, only six small spots were 
visible, though the same telescope was used, and circum- 
stances were equally favorable. 

The sun is a difficult object to view through a telescope, even when the eye is pro- 
tected in the best manner by colored glasses. In some cases (as in one related to the 
author by Professor Caswell, of Brown University), the heat becomes so great as to 
spoil the eye-pieces of the instrument, and sometimes the eye of the observer is irrepa- 
rably injured. 

314. The solar spots are all found within a zone 60° 
wide — i. e.) 30° each side of the sun's equator. They are 
generally permanent, though they have been known to 

318. View of sun's surface through telescopes ? Number of spots seen? 
Are they always to he seen? How from 1611 to 1G29 ? In 1825? Prof. 
Olmsted's statement? How from 1650 to 1670? From 1676 to 1684? In 
1846 ? (What said of difficulties of observing?) 

314. Where are these spots situated ? Are they permanent? What mo- 




148 ASTRONOMY. 



break in pieces, and disappear in a very short time. 
They sometimes break out again in the same places, or 
where none were perceptible before. They pass from 
left to right over the sun's disk in 13 days, 15 hours, 
and 45 minutes ; from which it has been ascertained that 
he performs a sidereal revolution on his axis, from west 
to east, or in the di- 
rection of all the 
planets, every 25 e 
days, 7 hours, and * i 



BIDEEEAL AND SYNODIC REVOLUTIONS OF THE SUN. 




48 minutes. /. m -^o H 

1. His apparent or synodio if 

revolution requires 27 days 7£ j 1 £V 

hours; but this is as much more i * o/Tl^:. tV^ 

than a complete revolution upon \ cvV\0^ c -^ ""' 

his axis, as the earth has ad- • ..--"" 

vanced in her orbit in 25 days 8 iH&n 

hours. Let S represent the sun, ?apC 

and A the earth in her orbit. 

When she is at A, a spot is seen 

upon the disk of the sun at B. 

The sun revolves in the direction of the arrows, and in 25 days 10 hours the spot comes 

round to B again, or opposite the star E. This is a sidereal revolution. 

2. During "these 25 days 8 hours, the earth has passed on in her orbit some 25°, or 
nearly to C, which will require nearly two days for the spot at B to get directly toward 
the earth, as shown at D. This last is a synodic revolution. It consists of one com- 
plete revolution of the sun upon his axis, and about 27° over. 

315. Of the nature of these wonderful spots, a variety 
of opinions have prevailed, and many curious theories 
have been constructed. Lalande, as cited by Herschel, 
suggests that they are the tops of mountains on the sun's 
surface, laid bare by fluctuations in his luminous atmos- 
phere ; and that the penumbrse are the shoaling declivi- 
ties of the mountains, where the luminous fluid is less 
deep. Another gentleman, of some astronomical knowl- 
edge, supposes that the tops of the solar mountains are 
exposed by tides in the sun's atmosphere, produced by 
planetary attraction. 

To the theory of Lalande, Dr. Herschel objects that 
it is contradicted by the sharp termination of both the in- 
ternal and external edges of the penumbrse ; and ad- 
vances as a more probable theory, that " they are the 

tion have they? What conclusion from it? (What revolution is this? 
What time required for a synodic revolution ? Illustrate.) 

315. What are these spots supposed to be ? Lalande ? &c. Dr. Herschel's 
remark ? Prof Olmsted ? Prof. Wilson ? Experiments of Prof. Henry I 



OF THE SUN. 149 



dark, or, at least, comparatively dark, solid body of the 
sun itself, laid bare to our view by those immense fluc- 
tuations in the luminous regions of the atmosphere, to 
which it appears to be subject." Prof. Olmsted supports 
this theory by demonstrating that the spots must be 
"nearly or quite in contact with the body of the sun." 

In 1773, Prof. Wilson, of the University of Glasgow, 
ascertained, by a series of observations, that the spots 
were probably " vast excavations in the luminous matter 
of the sun ;" the nuclei being their bottom, and the um- 
brae their shelving sides. This conclusion varies but 
little from that of Dr. Herschel, subsequently arrived at. 

In a series of experiments conducted by Prof. Henry, 
of Princeton, by means of a thermo-electrical apparatus, 
aj3plied to an image of the sun thrown on a screen from 
a dark room, it was found that the spots were perceptibly 
colder than the surrounding light surface. 

316. The magnitude of the solar spots is as variable 
as their number. Upon this point, the second cut pre- 
ceding gives a correct idea, as it is a pretty accurate rep- 
resentation of the sun's disk, as seen by the writer on the 
22cl of September, 1846. In 1799, Dr. Herschel ob- 
served a spot nearly 30,000 miles in breadth ; and he 
further states, that others have been observed, whose 
diameter was upward of 45,000 miles. Dr. Dick ob- 
serves that he has several times seen spots which were 
not less than ^ of the sun's diameter, or 22,192 miles 
across. 

It is stated, upon good authority, that solar spots have 
been seen by the naked eye — a fact from which Dr. 
Dick concludes that such spots could not be less than 
50,000 miles in diameter. The observations of the 
writer, as above referred to, and represented in the cut, 
would go to confirm this deduction, and to assign a still 
greater magnitude to some of these curious and interest- 
ing phenomena. 

317. The axis of the sun is inclined to the ecliptic 7J°, 

316. What said of the size of the solar spots? Dr. Herschel's observa- 
tions ? Dr. Dick's ? The writer's ? 



150 ASTRONOMY. 



or, more accurately, 7° 20'. This is but a slight deviation 
from what we may call a perpendicular ; so that, in rela- 
tion to the earth, he may be considered as standing up 
and revolving with one of his poles resting upon a point, 
just half his diameter below the ecliptic. 

As the result of the sun's motion upon his axis, his 
spots always appear first on his eastern limb, and pass off 
or disappear on the west. But though the direction of 
the spots, as viewed from the earth, is from east to west, 
it only proves his motion to coincide with that of the 
earth, which we call from west to east ; as when two 
spheres revolve in the sa?ne direction, the sides toward 
each other will appear to move in opposite directions. 
During one-half of the passage of the spots across the 
sun's disk, their apparent motion is accelerated; and 
during the remainder, it is retarded. 

This apparent irregularity in the motion of the spots 
upon the sun's surface, is the necessary result of an 
equable motion upon the surface of a globe or sphere. 
When near the eastern limb, the spots are coming partly 
toward us, and their angular motion is but slight ; but 
when near the center, their angular and real motions are 
equal. So, also, as the spots pass on to the west, it is 
their angular motion only that is diminished, while the 
motion of the sun upon his axis is perfectly uniform. 

318. The figure of the sun affects not only the appa- 
rent velocity of the spots, but also their forms. When 
first seen on the east, they appear narrow and slender, as 
represented in the cut, page 147. As they advance 
westward, they continue to widen or enlarge till they 
reach the center, where they appear largest; when they 
again begin to contract, and are constantly diminished, 
till they disappear. 

319. Another result of the revolution of the sun upon 
an axis inclined to the ecliptic, and the revolution of the 

817. How is the sun's axis situated? What said of the direction of the 
spots ? Of their rate of motion ? 

318. Of the cause of this irregularity? What variations m^i& forms of 
the solar spots ? Cause ? 

819. What other result of the sun's revolution about an inclined axis? 
(Illustrate by diagrams.) 



OF THE SUN. 



151 



earth around him, is, that when viewed from our mov- 
able observatory, the earth, at different seasons of the 
year, the direction of the spots seems materially to vary. 



VARIOUS DIRECTIONS OF THE SOLAR 6POTS. 




March. 



June. 



September. 



December. 



1. Let EF represent the plane of the ecliptic. In March, the spots describe a curve, 
which is convex to the south, as shown at A. In June, they cross the sun's disk in 
nearly straight lines, but incline upward. In September, they curve again, though in 
the opposite direction; and in December, pass over in straight lines, inclining down- 
ward. The figures B and D show the inclination of the sun's axis. 

2. The cause of this difference in the direction of the solar spots will be fully under- 
stood by the following diagram : 

SOLAR SPOTS OBSERVED FROM DIFFERENT POINTS. 



DEC, 




OUNEi 



MARCH 



Let the student imagine himself stationed upon the earth at A, in March, looking 
upon the sun in the center, whose north or upper pole is now inclined toward him. 
The spots will then curve downward. Three months afterward — viz., in June — the 
earth will be at B; when the sun's axis will incline to the left, and the spots seem to 
pass upward to the right. In three months longer, the observer will be at C, when the 
north pole of the sun will incline from him, and the spots seem to curve upward ; and 
in three months longer, he will be at D, when the axis of the sun will incline to the 
right, and the spots seem to incline downward. 

320. Of the physical constitution of the sun, very lit- 
tle is known. When seen through a telescope, it is like 
a globe of fire, in a state of violent commotion or ebu- 
lition. La Place believed it to be in a state of actual 
combustion, the spots being immense caverns or craters, 
caused by eruptions or explosions of elastic fluids in the 
interior. 



320. What said of the physical constitution of the sun ? La Place's opin- 
ion ? Most probable opinion ? 



152 ASTRONOMY. 



The most probable opinion is, that the body of the sun 
is opake, like one of the planets ; that it is surrounded 
by an atmosphere of considerable depth ; and that the 
light is sent off from a luminous stratum of clouds, float- 
ing above or outside the atmosphere. This theory accords 
best with his density, and with the phenomena of the 
solar spots. 

321. Of the temperature of the sun's surface, Dr. Her- 
schel thinks that it must exceed that produced in fur- 
naces, or even by chemical or galvanic processes. By 
the law governing the diffusion of light, he shows that 
a body at the sun's surface must receive 300,000 times 
the light and heat of our globe ; and adds that a far less 
quantity of solar light is sufficient, when collected in the 
focus of a burning-glass, to dissipate gold and platina into 
vapor. The same writer observes that the most vivid 
flames disappear, and the most intensely ignited solids 
appear only as black spots on the disk of the sun, when 
held between him and the eye. From this circumstance 
he infers, that however dark the body of the sun may 
appear, when seen through its spots, it may, neverthe- 
less, be in a state of most intense ignition. It does not, 
however, follow, of necessity, that it ?nust be so. The 
contrary is, at least, physically possible. A perfectly 
reflective canopy would effectually defend it from the 
radiation of the luminous regions above its atmosphere, 
and no heat would be conducted downward through a 
gaseous medium increasing rapidly in density. The 
great mystery, however, is to conceive how so enormous 
a conflagration (if such it be) can be kept up from age 
to age. Every discovery in chemical science here leaves 
us completely at a loss, or rather seems to remove further 
from us the prospect of explanation. If conjecture 
might be hazarded, we should look rather to the known 
possibility of an indefinite generation of heat by friction, 
or to its excitement by the electric discharge, than to 
any actual combustion of ponderable fluid, whether solid 
or gaseous, for the origin of the solar radiation. 

321. Sun's temperature ? Dr. Herschel's idea? What reasoning against 
his opinion ? What mystery ? 



THE ZODIACAL LIGHT. 



153 



ZODIACAL LIGHT. 




322. The Zodiacal Light is a faint nebulous light, re- 
sembling the tail of a comet, or the milky way, which 
seems to be reflected from 
the regions about the sun, 
and is distinguishable from 
ordinary twilight. Its form 
is that of a pyramid or 
cone, with its base toward 
the sun, and inclined slight- 
ly to the ecliptic. It seems 
to surround the sun on all 
sides, though at various 
depths, as it may be seen 
in the morning preceding 
the sun, as well as in the 
evening following him ; 
and the bases of the cones, 
where they meet at the sun, are much larger than his 
diameter. 

323. Of the nature of this singular phenomenon, very 
little is positively known. It was formerly thought to 
be the atmosphere of the sun. Prof. Nichol says : " Of 
this, at least, we are certain— the zodiacal light is a phe- 
nomenon precisely similar in kind to the nebulous atmos- 
pheres of the distant stars, &c." Sir John Herschel re- 
marks that it is manifestly of the nature of a thin len- 
ticularly-formed atmosphere, surrounding the sun, and 
extending at least beyond the orbit of Mercury, and 
even of Venus. He gives the apparent angular distance 
of its vertex from the sun, at from 40° to 90° ; and the 
breadth of its base, from 8° to 30°. It sometimes ex- 
tends 50° westward, and 70° east of the sun at the same 
time. 

324. The form of this substance surrounding the sun, 
and which is sufficiently dense to reflect his light to the 



822. What is the zodiacal light? Its form? When seen? 

323. Nature of this light 3 Former opinion ? Prof. Niehol's remark? Dr. 
Herschel's ? Its extent" from the sun 3 .- 

324. Form of this light? How situated with respect to sun s axis, &c. i 
(Illustrate by diagram.) 

7* 



154 



ASTRONOMY. 



being seen 



FORM,. EXTENT, ETC., OF THE ZODIACAL 
LIGHT. 




earth, seems to be that of a lens; or rather that of a 
huge wheel, thickest at the center, and thinned down 
to an edge at the outer extremities. Its 
edgewise, and only one-half 
at a time, gives it the ap- 
pearance of two pyramids 
with their bases joined at 
the sun. It is an interest- 
ing fact, stated by Prof. 
Nichol, that this light or 
nebulous body lies in the 
plane of the sun" s equator. 
A line drawn through its 
transverse diameter, or 
from one apex of the pyra- 
mids to the other, would 
cross the axis of the sun 
at rio;ht angles. This fact 
would seem to indicate a revolution of this curious sub- 
stance with the sun upon his axis. 

Let A, in the above cut, represent the sun, B B his axis ; then C C will represent tho 
extent, and D D the thickness of this curious appendage. 

325. In regard to its atmospheric character, Dr. Dick 
observes that this opinion now appears extremely dubi- 
ous ; and Prof. Olmsted refers to La Place, as showing 
that the solar atmosphere could never reach so far from 
the sun as this light is seen to extend. 

Another class of astronomers suppose this light, or 
rather the substance reflecting this light, to be some of 
the original matter of which the sun and planets were 
composed — a thin nebulous substance in a state of con- 
densation, and destined either to be consolidated into new 
planetary worlds, during the lapse of coming ages, or to 
settle down upon the sun himself as a part of his legiti- 
mate substance. This theory will be noticed again when 
we come to speak of Nebulae and Nebulous Stars. 

326. After all the observations that have been made, 



325. Remark of Dr. Dick respecting its atmospheric character? Olmsted 
and La Place ? What other the or v ! K 



THE SUN'S PROPER MOTION IN SPACE. 155 



and the theories that have been advanced, it must be ad- 
mitted that the subject of the zodiacal light is but imper- 
fectly understood. Prof. Olmsted supposes it to be a 
nebulous body, or a thin vapory mass revolving around 
the sun ; and that the meteoric showers which have oc- 
curred for several years in the month of November, may 
be derived from this body. This is the opinion of Arago, 
Biot, and others. 

The best time for observing the zodiacal light is on 
clear evenings, in the months of March and April. It 
may be seen, however, in October, Xovember, and De- 
cember, before sunrise ; and. also in the evening sky. 

the sun's motion in space. 

327. Although, in general terms, we speak of the sun 
as the fixed center of the system, it must not be under- 
stood that the sun is absolutely without motion. On the 
contrary, he has a periodical motion, in nearly a circular 
direction, around the common center of all the planetary 
bodies ; never deviating from his position by more than 
twice his diameter. From the known laws of gravita- 
tion, it is certain that the sun is affected in some measure 
by the attraction of the planets, especially when many 
of them are found on the same side of the ecliptic at the 
same time : but this would by no means account for so 
great a periodical motion. 

328. In addition to the motion above described, the 
sun is found to be moving, with all his retinue of planets 
and comets, in a vast orhit, around some distant and 
hitherto unknown center. This opinion was first ad- 
vanced, we think, by Sir William Herschel ; but the 
honor of actually determining this interesting fact be- 
longs to Struve, who ascertained not only the direction 
of the sun and solar system, but also their velocity. 

326. Is this subject well understood as yet? Prof. Olmsted's theory? 
When the best time for observing the zodiacal light ? 

327. Is the sun really stationary ? What motion ? How affected bv plan- 

828. What other motion ? Who first advanced the opinion that he hud 
such a motion ' Who demonstrated it ? Toward what point is the sun and 



156 ASTRONOMY. 



The point of tendency is toward the constellation Her- 
cules, right ascension 259°, declination 35°. The ve- 
locity of the sun in space is estimated at 8 miles per 
second, or 28,000 miles per hour. Its period is about 
18,200,000 years ; and the arc of its orbit, over which 
the sun has traveled since the creation of the world, 
amounts to only about goVo^ P ar ^ °f ^is orbit, or seven 
degrees — an arc so small, compared with the whole, as 
to be hardly distinguishable from a straight line. 

329. "With this wonderful fact in view, we may no 
longer consider the sun as fixed and stationary, but rather 
as a vast and luminous planet, sustaining the same rela- 
tion to some central orb that the primary planets sustain 
to him, or that the secondaries sustain to their primaries. 
Nor is it necessary that the stupendous mechanism of 
nature should be restricted even to these sublime propor- 
tions. The sun's central body may also have its orbit, 
and its center of attraction and motion, and so on, till, 
as Dr. Dick observes, we come to the great center of all 

tO the THRONE OF GoD ! 

Professor Madler, of Dorpat, in Russia, has recently announced as a discovery that 
the star Alcyone, one of the seven stars, is the center around which the sun and soiar 
system are revolving. 



CHAPTER XI. 



MISCELLANEOUS REMARKS UPON THE SOLAR SYSTEM. 



NEBULAR THEORY OF THE ORIGIN OF THE SOLAR SYSTEM. 

330. It w r as the opinion of La Place, a celebrated 
French astronomer, that the entire matter of the solar 
system, which is now mostly found in a consolidated 

solar system tending ? Its velocity ? Period of revolution ? Amount of its 
progress since the creation of the world ? 

329. How, then, should the sun be considered ? How extend the analogy ? 
W T hat further recent discovery, and by whom ? 

330. State the " nebular theory" of the origin of the solar system? Who 
first started this theory ? 



ORIGIN OF THE SOLAR SYSTEM NEBULAR THEORY. 157 



state, in the sun and planets, was once a vast nebula or 

faseous vapor, extending beyond the orbits of the most 
istant planets — that in the process of gradual conden- 
sation, by attraction, a rotary motion was engendered 
and imparted to the whole mass — that this motion caused 
the consolidating matter to assume the form of various 
concentric rings, like those of Saturn ; and, finally, that 
these rings collapsing, at their respective distances, and 
still retaining their motion, were gathered up into plan- 
ets, as they are now found to exist. This opinion is sup- 
posed to be favored, not only by the fact of Saturn's 
revolving rings, but by the existence of the zodiacal light, 
or a resisting medium about the sun ; and also by the 
character of irresolvable or planetary nebulas, hereafter 
to be described. * 

331. To this theory, however, there are many plau- 
sible, if not insurmountable, objections. 

(a.) It seems to be directly at variance with the Mosaic 
account of the creation of the sun, moon, and stars. 
The idea that the sun and all the planets were made up, 
so to speak, out of the same general mass, not only 
throws the creation of this matter back indefinitely into 
eternity, but it substitutes the general law of attraction 
for the more direct agency of the Almighty. The crea- 
tion spoken of in the Bible thus becomes not the origi- 
nating of things that did not previously exist, but the 
mere organization or arrangement of matter already 
existing. 

(b.) The supposed consolidation of the nebulous mass, 
in obedience to the general law of attraction, does not 
of itself account for the rotary motion which is an essen- 
tial part of the theory. Under the influence of mere at- 
traction, the particles must tend directly toward the cen- 
ter of the mass, and consequently could have no tendency 
to produce a rotary motion during the process of conden- 
sation. 

(c.) The variation of the planetary orbits from the 



331. What said of it? State the first objection named? The second? 
Third ? Fourth ? Fifth ? What remark added by the author ? 

14 



15 S ASTBONOMY. 



plane of the sun's equator contradicts the nebular theory. 
If the several primary planets were successively thrown 
off from the general mass, of which the sun is a part, 
they could not have been separated from the parent body 
till they were near the plane of its equator. ]S~ow, as 
the sun is assumed to be a part of the same mass, re- 
volving still, the theory would require that the portions 
now separated from him, and called planets, should still 
revolve in the plane of his equator. But instead of this, 
it is found that some of them vary from this plane to the 
amount of near 42°. 

(d.) This theory assumes not only that the primary 
planets were thrown off from the parent mass by its 
rapid revolution, but that the primaries, in turn, threw 
oft' their respective satellites. These, ' then, should all 
revolve in the plane of the planetary equators respect- 
ively, and in the direction in which their primaries re- 
volve. But their orbits not only depart from the plane 
of the equators of their primaries (Jupiter's satellites 
excepted), but the moons of Uranus actually have a 
retrograde or backward revolution. 

(e.) If the sun and planets are composed of what was 
originally the same mass, it will be necessary to show 
why they differ so materially in their physical natures — 
why the sun is self-luminous, and the planets opake. 

But we have not room to discuss the subject at length 
in this treatise. It is but justice, however, to say, that 
men eminent for learning and piety have advocated 
the nebular ilieovy, in the belief that it is perfectly con- 
sistent with the Mosaic account of creation. But the 
writer is frank to state, that while he acknowledges the 
force of some of the considerations urged in its sup- 
port, he has not yet seen reason for adopting this theory 
of the origin of the solar system. " Through faith we 
understand that the worlds were framed by the word of 
God [not by the law of gravitation], so that things which 
are seen were not made of things which do appear [or of 
pre-existing matter]." — Heb. xi. 3. 

332. Upon the supposition that the sun and planets 
were created as they are, by the direct act of God, an 



WERE THE ASTEROIDS ORIGINALLY ONE PLANET? 159 



inquiry at once arises as to the probable extent of the 
creation recorded by Moses. Does it include the whole 
universe \ or is it to be understood as applicable only to 
the solar system ? Upon this point our only light is, that 
"in the beginning God created the heavens and the 
earth" — that he not only made the sun and moon, but 
that " he made the stars also ; ? ' and that when these were 
spoken into being, God had "finished" his work. (See 
Genesis, 1st chapter.) " Thus the heavens and the earth 
were finished, and all the host of them." It seems most 
probable, therefore, that the Mosaic creation includes the 
whole material universe — that when God "laid the foun- 
dations of the earth," and the "heavens were the work 
of his hands," he " made the worlds also ;" that is, they 
were then all " trained by the word of God.' 7 

WERE THE ASTEROIDS ORIGINALLY ONE PLANET ? 

333. Some very curious speculations have been enter- 
tained by astronomers in regard to the origin of the 
Asteroids. As in the case of the recently discovered 
planet, Xeptune, the existence of a large planet between 
the orbit of Mars and Jupiter was suspected before the 
asteroids were known. This suspicion arose mainly from 
the seeming chasm that the absence of such a body would 
leave in the otherwise well-balanced solar system. The 
prediction that such a body would be discovered in the 
future stimulated the search of astronomers, till at length, 
instead of one large planet, eighteen small ones have, 
one after another, been discovered. 

331. From certain peculiarities of the asteroids, it has 
been considered highly probable that they are the frag- 
ments of one large planet, which has been burst asunder 
by some great convulsion or collision. The grounds of 
this opinion are as follows : 

332. What other interesting question started ? What light upon this sub- 
ject ? What most probable ! 

333. What curious speculation respecting the asteroids \ What suspicions 
before any of them were discovered \ 

334. What opinion respecting the origin of the asteroids i State the 
grounds of this opinion in order. 



160 ASTRONOMY, 



(a.) The asteroids are much, smaller than any of the 
other primary planets. .- 

(b.) They are all at nearly the same distance from the 
sun. 

(c.) Their periodic revolutions are accomplished in 
nearly the same time. The difference of their periodic 
times is not greater than might result from the supposed 
disruption, as the parts thrown forward would have their 
motion accelerated, while the other parts would be thrown 
bach or retarded ; thus changing the periodic times of 
both. 

(d.) The great departure of the orbits of the asteroids 
from the plane of the ecliptic is supposed to favor the 
hypothesis of their having been originally one planet, the 
assumption being that the explosion separating the ori- 
ginal body into fragments would not only accelerate some 
portions and retard others, but would also throw them out 
of the plane of the original orbit, and in some cases still 
further from the ecliptic. 

(e.) Their orbits are more eccentric than those of the 
other primaries. Although the tables show the eccen- 
tricity of Uranus's orbit as greater in miles than that of 
even Juno or Pallas, yet when we consider the difference 
in the magnitude of their orbits, it will easily be seen 
that his orbit is less elliptical than theirs. 

(f.) The orbits of Ceres and Pallas, at least, cross eacli 
other. This, if w r e except, perhaps, the orbits of some 
of the comets, is a perfect anomaly in the solar system. 

335. From all these circumstances, it has been con- 
cluded that the asteroids are onlv the fragments of an 
exploded world, which have assumed their present forms 
since the disruption, in obedience to the general laws of 
gravitation. This theory, first advanced by Dr. Olbers, is 
favored by Prof. Nichol, Dr. Brewster, Dr. Dick, and 
others ; while Sir John Herschel observes that it may 
serve as a specimen of the dreams in which astronomers, 
like other speculators, occasionally and harmlessly in- 

335. W T ho was the author of this theory ? What distinguished astrono- 
mers favor it ? What says Sir John Herschel ? Remark of Dr. Dick ? Opin- 
ion and remarks of the author ? 



ARK THE PLANETS INHABITED % 161 



dulge. Dr. Dick remarks that the breaking up of the 
exterior crust of the earth, at th$ time of the general 
deluge, was a catastrophe as tremendous and astonishing 
as the bursting asunder of a large planet. In view, how- 
ever, of the harmony and order that everywhere reign 
throughout the planetary regions, directing the pathway 
and controlling the destiny of every world, it is hard to 
believe either that one world has been so constructed 
as to explode, like a vast bomb-shell, and scatter its frag- 
ments over the regions of its former pathway; or that 
He who guides even the erratic comet has allowed a pon- 
derous world to get so off its track, as to dash itself to 
pieces against its fellow worlds. 

ARE THE PLANETS INHABITED BY RATIONAL BEINGS I 

o?j(j. Upon this interesting question, it must be ad- 
mitted that we have no positive testimony. The argu- 
ment for the inhabitedness of the planets rests wholly 
upon analogy, and the conclusion is to be regarded only 
in the light of a legitimate inference. Still, it is remark- 
able that those who are best acquainted with the facts of 
astronomy are most confident that other worlds as well 
as ours are the abodes of intellectual life. Indeed, as 
Dr. Dick well remarks, it requires a minute knowledge 
of the whole scenery and circumstances connected with 
the planetary system, before this truth comes home to the 
understanding with full conviction. 

337. The analogies from which it is concluded that all 
the primary planets, at least, are inhabited by rational 
beinirs, are the following : 

(a.) The planets are all solid lodies resembling the 
earth, and not mere clouds or vapors. 

(b ) They all have a spherical or spheroidal figure, like 
our own planet. 

(c.) The laws of gravitation, by which we are kept 
upon the surface of the earth, prevail upon all the other 

886. What other question proposed? What admission'- Nature of the 
evidence of the inhabitedness of the planets ? What remarkable fact I Re- 
mark of Dr. Dick ? 

337. State the principal points of analogy between our globe and the other 

14* 



162 ASTRONOMY. 



planets, as if to bind races of material beings to their sur- 
faces, and provide fof the erection of habitations and 
other conveniences of life. It is very remarkable, how- 
ever, that those planets whose bulks are such as to indi- 
cate an insupportable attractive force, are not only less 
dense than our globe, but they have the most rapid daily 
revolution ; as if, by diminished density, and a strong 
centrifugal force combined, to reduce the attractive force, 
and render locomotion possible upon their surfaces. 

(d.) The magnitudes of the jDlanets are such as to af- 
ford ample scope for the abodes of myriads of inhabit- 
ants. It is estimated that the solar bodies, exclusive of 
the comets, contain an area of 78,000,000,000 of square 
miles, or 397 times the surface of our globe. According 
to the population of England, this vast area would afford 
a residence to 21,875,000,000,000 of inhabitants; or 
27,000 times the population of our globe. 

(e.) The planets have a diurnal revolution around 
their axes, thus affording the agreeable vicissitudes of 
day and night. Not only are they opake bodies like our 
globe, receiving their light and heat from the sun, but 
they also revolve so as to distribute the light and shade 
alternately over each hemisphere. There, too, the glo- 
rious sun arises, to enlighten, warm, and cheer ; and there 
" the sun-strown firmament" of the more distant heavens 
is rendered visible by the no less important blessing of a 
periodic night. 

(/'.) All the planets have an annual revolution round 
the sun ; which, in connection with the inclination of 
their axes to their respective orbits, necessarily results in 
the production of seasons. 

(g.) The planets, in all probability, are enveloped in 
atmospheres. That this is the case with many of them 
is certain ; and the fact that a fixed star, or any other 
orb, is not rendered dim or distorted when it approaches 
their margin, is no evidence that the planets have no at- 
mosphere. This appendage to the planets is known to 
vary in density ; and in those cases where it is not de- 
planets. Substance? Forms? Gravitation? Magnitude? Days and 
nights? Seasons? Atmospheres? Moons? Mountains? &c. 



ARE THE PLANETS INHABITED? 163 



tected by its intercepting or refracting the light, it may 
be of a nature too clear and rare to produce such phe- 
nomena. 

(h.) The principal primary planets are provided with 
moo?is or satellites, to afford them light in the absence of 
the sun. It is not improbable that both Mars and Yenus 
have each, at least, one moon. The earth has one ; and 
as the distances of the planets are increased, the number 
of moons seems to increase. The discovery of six around 
Uranus, and only one around Neptune, is no evidence 
that others do not exist which have not yet been dis- 
covered. 

(i.) The surfaces of all the planets, primaries as well 
as secondaries, seem to be variegated with hill and dale, 
mountain and plain. These are the spots revealed by 
the telescope. 

(j.) Every part of the globe we inhabit is destined 
to the support of animal life. It would, therefore, be 
contrary to the analogy of nature, as displayed to us, to 
suppose that the other planets are empty and barren 
wastes, utterly devoid of animated being. And if ani- 
mals of any kind exist there, why not intelligent beings ? 

338. If other worlds are not the abodes of intellectual 
life, for what were they created ? What influence do 
they exert upon our globe, especially those most remote? 
There are doubtless myriads of worlds beyond our system 
that will never even be seen by mortal eye, and that have 
no perceptible connection with our globe. If, then, they 
are barren and uninhabited islands in the great ocean of 
immensity, we repeat, for what were they created ? The 
inquiry presses itself upon the mind with irresistible 
force, Why should this one small world be inhabited, 
and all the rest unoccupied ? For what purpose were all 
these splendid and magnificent worlds fitted up, if not to 
be inhabited ? Why these days and years — this light and 
shade — these atmospheres, and seasons, and satellites, and 
hill and dale ? 



333. What difficulty on the supposition that the planets are not inhab- 
ited? 



164 ASTRONOMY. 



339. To suppose all these worlds to be fitted up upon 
one general plan, provided with similar conveniences as 
abodes for intellectual beings, and yet only one of them 
to be inhabited, is like supposing a rich capitalist would 
build some thirty fine dwellings, all after one model, 
though of different materials, sizes, and colors, and pro- 
vide in all for light, warmth, air, &c. ; and yet, having 
placed the family of a son in one of them, allow the 
remaining twenty-nine to remain unoccupied forever ! 
And as God is wiser than man, in the same proportion 
does it appear absurd, that of the twenty-six planetary 
temples now known to exist, only one has ever been occu- 
pied ; while the remainder are mere specimens of Divine 
architecture, wheeling through the solitudes of immen- 
sity ! The legitimate and almost inevitable conclusion, 
therefore, is, that our globe is only one of the many 
worlds which God has created to be inhabited, and which 
are now the abodes of his intelligent offspring. It seems 
irrational to suppose that we of earth are the only intel- 
ligent subjects of the " Great King,' 5 whose dominions 
border upon infinity. It is much more in keeping with 
sound reason, and with all the analogies of our globe, 
to suppose that 

" Each revolving sphere, a seeming point, 
Which through night's curtain sparkles on the eye, 
Sustains, like this our earth, its busy millions." 

340. The fact that we neither see, nor hear, nor hear 
from the inhabitants of other worlds, is no evidence that 

such inhabitants do not exist. It would have been 
premature in Columbus had he concluded, when he saw 
land in the distance, that it was uninhabited, simply be- 
cause he could not hear the shout of its savages, or see 
them gathered in groups upon the beach. So in regard 
to the distant planets. Our circumstances forbid our 
knowing positively that they are inhabited ; so that the 
absence of that knowledge is no argument against the 
inhabitedness of other worlds. 

839. What illustration ? Conclusion? Poetry? 

340. What said of the objection that we neither see, hear, nor hear from 
the inhabitants of the other worlds ? 



ARE THE PLANETS INHABITED? 165 



341. It may be thought that the extremes of heat and 
cold on some of the planets must be fatal to the idea of 
animal life, at least. But even this does not follow. 
Upon our globe, some animals live and flourish where 
others would soon die from heat or cold. And some ani- 
mals, having cold blood, may be frozen, and yet live. 
So in other worlds. He who made the three Hebrews 
to live in the fiery furnace, can easily adapt the inhabit- 
ants of Mercury to their warm abode. And of the exte- 
rior planets we have only to say : 

" W T ho there inhabit must have other powers, 
Juices, and veins, and sense, and life, than ours ; 
One moment's cold, like theirs, would pierce the bone, 
Freeze the heart's blood, and turn us all to stone !" 

Adaptation is a law of the universe ; and this at once 
obviates every difficulty in regard to the temperature of 
the planets, which might otherwise be urged as a reason 
why they were not inhabited. 

341. Objection drawn from extremes of temperature? Poetry? What 
great law answers every such objection? 



PART III. 

THE SIDEREAL HEAVENS. 



C HA PTER I . 

THE FIXED STARS CLASSIFICATION, NUMBER, DISTANCE, ETC. 

342. The sidereal heavens embrace all those celestial 
bodies that lie around and beyond the solar system, in 
the region of the fixed stars. 

The fixed stars are distinguished from the planetary 
bodies by the following characteristics : 

(a.) They shine by their oxen light, like the sun, and 
not by- reflection. 

(5.) To the naked eye, they seem to twinkle or scintil- 
late / while the planets appear tranquil and serene. 

(c.) They maintain the same general positions, with 
respect to each other, from age to age. On this account, 
they are called fixed stars. 

(d.) They are inconceivably distant; so that, when 
viewed through a telescope, they present no sensible disk, 
but appear only as shining points on the dark concave of 
the sky. To these might be added several other peculi- 
arities, which will be noticed hereafter. 

343. For purposes of convenience, in finding or refer- 
ring to particular stars, recourse is had to a variety of 
artificial methods of classification. 

342. What parts of the book have we now gone over? Upon what do we 
now enter ? What is meant by the sidereal heavens ? How are the fixed 
Btara distinguished from planetary bodies ? 

343. What are constellations ? Their origin ? 



THE FIXED STARS CLASSIFICATION. 



167 



First, The whole concave of the heavens is divided 
into sections or groups of stars of greater or less extent. 
The ancients imagined that the stars were thrown toge- 
ther in clusters, resembling different objects, and they 
consequently named the different groups after the objects 
which they supposed them to resemble. These clusters, 
when thus marked out by the figure of some animal, 
person, or thing, and named accordingly, were called 
constellations. 

344. Secondly, The stars are all classed according to 
their magnitudes. There are usually reckoned twelve 
different magnitudes, of which the first six only are 
visible to the naked eye, the rest being telescopic stars. 
These magnitudes, of course, relate only to their apparent 
brightness ; as the faintest star may appear dim solely on 
account of its immeasurable distance. The method by 
which stars of different magnitudes are distinguished in 
astronomical charts is as follows : 



STAES OP DIFFEEENT MAGNITUDES. 

3 4 5 6 7 



8 9 10 11 12 



>JC ^ ^ * * 



" It must be observed," says Dr. Herschel, " that this classification into magnitudes is 
entirely arbitrary. Of a multitude of bright objects, differing, probably, intrinsically 
both in size and in splendor, and arranged at unequal distances from us, one must of 
necessity appear the brightest; the one next below it brighter still, and so on.* 1 

345. The next step is to classify the stars of each con- 
stellation according to their magnitude in relation to each 
other, and without reference to other constellations. In 
this classification, the Greek alphabet is first used. For 
instance, the largest star in Taurus would be marked (a) 
Alpha ; the next largest (fi) Beta ; the next (7) Gamma, 
&c. When the Greek alphabet is exhausted, the Roman 
or English is taken up ; and when these are all absorbed, 
recourse is finally had to figures. 

As Greek letters so frequently occur in catalogues and mnps of the stars, and on the 
celestial globes, the Greek alphabet is here inserted, for the benefit of those who are not 

344. How classified by magnitudes t (Remark of Dr. Herschel V) 

345. Next step in classifying 3 How conducted? Greek letters ? (Kepeat, 
the ttlphabet.) 



168 



ASTRONOMY. 



acquainted with it; but as the capitals are seldom used for designating the stars, the 
small characters only are given : 

THE GREEK ALPHABET. 



a 


Alpha 


a 


V 


Nu 


n 





Beta 


b 


k 


Xi 


X 


y 


Gamma 


g 





micron 


o short 


6 


Delta 


cl 


r 


Pi 


p 


t 


Epsilon 


e short 


p 


Rho 


r 


5 


Zeta 


z 


( 


Sigma 


s 


V 


Eta 


e long 


T 


Tau 


t 


e 


Theta 


th 


V 


Upsilon 


u 


i 


Iota 


i 





Phi 


ph 


K 


Kappa 


k 


X 


Chi 


ch 


A 


Lambda 


1 


* 


Psi 


ps 


^ 


Mu 


m 


to 


Omega 


o long 



34:6. To aid in finding particular stars, and especially 
in determining their numbers, and detecting changes, 
should any occur, astronomers haye constructed cata- 
logues of the stars, one of which is near 2,000 years old. 
Several of the principal stars have a specific name like 
the planets — as Sirius, Aldebaran, Hegulus, &c. ; and 
clusters of stars in a constellation sometimes receive a 
specific name, as the -Pleiades and Hyacles in Taurus. 

347. The stars are still further distinguished into 
double, triple, and quadruple stars, binary system, vari- 
able stars, periodic stars, nebulous stars, &c. ; all of 
which will be noticed hereafter. 

NUMBER OF THE FIXED STARS. 

348. The actual number of the stars is known only to 
Him who " telleth the number of the stars, and calleth 
them all by their names." The powers of the human 
mind are barely sufficient to form a vague estimate of 
the number near enough to be seen by our best tele- 
scopes, and here our inquiries must end. 

The number of stars, down to the twelfth magnitude, 
has been estimated as follows : 



346. What further methods for finding particular stars ? 

347. How are the stars still further distinguished ? 

348. Number of the stars? Of each magnitude? Number visible to 
naked eye? Additional seen through telescopes? Total? Remarks of 
Herschel and Olmsted ? 



NUMBER OF THE FIXED STARS. 



169 



Visible to the naked eye. 


Visible only th 


rough telescopes. 


1st magnitude . 18 


7th 3 


magnitude 26,000 


2d " . 52 


8th 


cc 


170,000 


3d " . 177 


9th 


t< 


1,100,000 


4th " . 376 


10th 


u 


7,000,000 


5th " . 1,000 


11th 


u 


46,000,000 


6th ". . 4,000 


12th 
Grar 


id total, 


300,000,000 


otal 5,623 


354,301,623 



NUMBER OF STARS OF EACH MAGNITUDE. 



Of these stars, Dr. Herschel remarks that from 15,000 
to 20,000 of the first seven magnitudes are already regis- 
tered, or noted down in catalogues ; and Prof. Olmsted 
observes that Lalande has registered the positions of no 
less than 50,000. 

3-19. The reason why there are so many more of the 
small stars than of the large ones is, that we are in the 
midst of a great cluster, with but few stars near us, the 
number increasing as the 
circumference of our 
view is enlarged. (See 
second cut, page 28, and 
also the adjoining.) 

Let the central star represent the 
sun (a star only among the rest), with 
the solar system revolving between 
him and the first circle. The IS stars 
in space 1st will appear to be of the 
first magnitude, on account of their 
nearnei*. and they are thus few be- 
cause they embrace but a small part 
of the entire cluster. The stars of 
space 2d will appear smaller, being 
more distant ; but as it embraces more 
space, they will be more numerous. 
Thus as we advance from one circle 
to another, the apparent magnitude 
constantly diminishes, but the num- 
ber constantly increases. The large 
white circle marks the limit of our 
natural vision. Even this cut fails to present fully to the eye the cause of the rapid in- 
crease in numbers, for we can only show the surface of a cut section of our firmament 
of stars, which exhibits the increase in a plane only; whereas our sun seems to be im- 
bedded in the midst of a magnificent cluster (like a single apple in the midst of a large 
tree richly laden with fruit), the stars of which we view around us in every direction. 




S4'i*. Why so many more of small stars than of the larger r (Illustrate by 
diagram. Does this convey a complete idea of the position of the sun, with 
reference to the fixed stars'? Why not ? What does his position more nearly 
resemble ?) , 

8 



170 ASTRONOMY. 



350. If we suppose that each of these suns is accom- 
panied only by as many planets as are embraced in our 
solar system, we have nine thousand millions of worlds 
in our firmament. No human mind can form a concep- 
tion of this number ; but even these, as will hereafter be 
shown, form but a minute and comparatively insignifi- 
cant portion of the boundless empire which the Creator 
has reared, and over which he reigns. " Lo, these are 
parts of his ways ; but how little a portion is heard of 
Him? but the thunder of his power who can under- 
stand." (Job xxvi. 14.) 

DISTANCES AND MAGNITUDES OF THE STARS. 

351. It has been demonstrated that the nearest of the 
fixed stars cannot be less than 20,000,000,000 {twenty 
hillions) of miles distant ! For light to travel over this 
space, at the rate of 200,000 miles per second, would re- 
quire 100,000,000 seconds, or upwards of three years. 

What, then, must be the distances of the telescopic 
stars, of the 10th and 12th magnitudes ? " If we admit," 
says Dr. Herschel, " that the light of a star of each mag- 
nitude is half that of the magnitude next above it, it will 
follow that a star of the first magnitude will require to 
be removed to 362 times its distance, to appear no larger 
than one of the twelfth magnitude. It follows, therefore, 
that among the countless multitude of such stars, visible 
in telescopes, there must be many whose light has taken 
at least a thousand years to reach us ; and that when we 
observe their places, and note their changes, we are, in 
fact, reading only their history of a thousand years' 
date, thus wonderfully recorded." Should such a star be 
struck out of existence now, its light would continue to 
stream upon us for a thousand years to come ; and should 
a new star be created in those distant regions, a thousand 
years must pass away before its light could reach the 
solar system, to apprise us of its existence. 



850. What supposition and conclusion ? Scripture quotation ? 
351. Distances of the nearest stars ? Time for light to travel over this 
space ? Suppositions and conclusions of Dr. Herschel ? 



MAGNITUDE OF THE STARS. 171 



352. From what we have already said respecting the 
almost inconceivable distances of the fixed stars, it will 
readily be inferred that they must be bodies of great 
magnitude, in order to be visible to us upon the earth. 
It is probable, however, that " one star diftereth from 
another" in its intrinsic splendor or " glory," although 
we are not to infer that a star is comparatively small be 
cause it appears small to us. 

353. The prevailing opinion among astronomers is, 
that what we call the fixed stars are so many suns and 
centers of other systems. From a series of experiments 
upon the light received by us from Sirius, the nearest oi 
the fixed stars, it is concluded that if the sun were re- 
moved 141,400 times his present distance from us, or 
to a point thirteen billions of miles distant, his light 
would be no stronger than that of Sirius ; and as Sirius 
is more than twenty billions of miles distant, he must, 
in intrinsic magnitude and splendor, be equal to two suns 
like ours. Dr. Wollaston, as cited by Dr. Herschel, con- 
cludes that this star must be equal in intrinsic light to 
nearly fourteen suns. According to the measurements 
of Sir Wm, Herschel, the diameter of the star Vega in 
the Lyre is 38 times that of the sun, and its solid con- 
tents 54,872 times greater! The star numbered 61 in 
the Swan is estimated to be 200,000,000 miles in di- 
ameter. 

354. Sir John Herschel states, that while making ob- 
servations with his forty-feet reflector, a star of the first 
magnitude was unintentionally brought into the field of 
view. " Sirius," says he, " announced his approach like 
the dawn of day ;" and so great was his splendor when 
thus viewed, and so strong was his light, that the great 
astronomer was actually driven from the eye-piece of his 
telescope by it, as if the sun himself had suddenly burst 
upon his view. 

352. What inference from the great distance of the stars ? What proba- 
bility as to the real magnitude of the stars? 

353. The prevailing opinion among" astronomers ? Conclusions from ex- 
periments with Sirius? Magnitude of Vega? Of No. 61 in the Swan ? 

354. Incident stated by Dr. Herschel? (Relative light of the stars of the 
first six magnitudes ?) 



172 ASTRONOMY. 



According to Sir Wm. Herschel, the relative light of the stars of the first six magni- 
tudes is as follows : 

Light of a star of the average 1st magnitude 100 

" « " 2d " 25 

« " " 3d " 12 

" " " 4th " 6 

" " " 5th u 2 

" 6th " 1 



CHAPTER II. 

DESCRIPTION OF THE CONSTELLATIONS. 

355. Although this work is designed particularly to 
illustrate the mechanism of the heavens, as displayed in 
the solar system, we are desirous of furnishing the 
learner with a sufficient guide to enable him to extend 
his inquiries and investigations not only to the different 
classes of bodies lying beyond the limits of the solar 
system in the far off heavens, but also to the constella- 
tions^ as such. For this purpose, we shall here furnish 
a brief description of the principal constellations visible 
in the United States, or in north latitude ; by the aid of 
which, the student will be able to trace them, with very 
little difficulty, upon that glorious celestial atlas which 
the Almighty has spread out before us. 

If the student will he at the trouble to identify the constellations by the aid of theso 
descriptions, and without the aid of charts, it will give him a practical familiarity with 
the heavens which can be acquired in no other way. Indeed, this exercise is indispen- 
sable to a competent knowledge of sidereal astronomy, even where maps of the constel- 
lations are used. Let all students, therefore, embrace every favorable opportunity for 
looking up the constellations. 

Those who wish to study their mythological history will consult the author's edition 
of the " Geography of the Heavens" by E. H. Burritt — the most reliable and popular 
work upon this subject in the English language. 

356. Of the nature and origin of the constellations 
we have already spoken, at 242. Their formation has 
been the work of ages. Some of them were known at 
least 3,000 years ago. In the 9th chapter of Job, we 

355. Principal design of this text-book ? "What further object ? What 
done for this purpose ? (Substance of note ?) 

356. What said of the formation of the constellations ? Antiquity ? 
Scripture allusions? 



DESCRIPTION OF THE CONSTELLATIONS. 173 



read of "Arcturus, Orion, and Pleiades, and the cham- 
bers of the south ;" and in the 38th chapter of the same 
book, it is asked, " Canst thou bind the sweet influences 
of Pleiades, or loose the bands of Orion ? Canst thou 
bring forth Mazzaroth in his season ? or canst thou guide 
Arcturus with his sons ?" 

357. The constellations are distinguished into ancient 
and modern. According to Ptolemy's catalogue, the 
ancients had only 48 constellations ; but being found 
convenient in the study of the heavens, new ones were 
added to the list, composed of stars not yet made up into 
hydras and dragons, till there is now scarcely stars 
or room enough left to construct the smallest new con- 
stellation, in all the spacious heavens. The present num- 
ber, according to the catalogue of the Observatory Royal 
of Paris, is 93. 

358. The constellations are further divided into the 
Zodiacal, Northern, and Southern. The zodiacal con- 
stellations are those which lie in the sun's apparent path, 
or along the line of the zodiac. The northern are those 
which are situated between the zodiacal and the north 
pole of the heavens ; and the southern, those w r hich lie 
between the zodiacal and the south pole of the heavens. 
They are distributed as follows — viz., 12 zodiacal, 35 
northern, and 46 southern. 

This division is convenient for reference ; but in tracing the constellations in the 
heavens, or upon a map. it is better to begin with those that are on or near the meridian, 
and proceed eastward, taking northern and southern together, so far as they are in view. 
And where classes in astronomy are organized during "the fall months, it will be found 
advantageous to begin with the constellations that are in view at seasonable hours during 
those months. 

359. In consequence of the eastward motion of the 
earth in its annual revolution, the constellations rise ear- 
lier and earlier every night ; so that if an observer were 
to w r atch the stars from the same position for a w T hole 
year, he would see each constellation, in turn, coming to 
the meridian at midnight (or at any other hour fixed 

357. How are the constellations classified ? How many of each ? In all ? 
. 358. How further classified ? Describe each. How many of each ? (What 
said in note ?) 

359. What said of the rising of the constellations? How proceed in de- 
scribing and tracing ? 



174 ASTRONOMY. 



upon), till he had seen the whole panorama of the heav- 
ens. Beginning, therefore, with the constellations that 
are on or near the meridian at 9 o'clock, on the 15th of 
November, and going eastward, we shall now proceed 
with our description of the constellations. 

OCTOBER, NOVEMBER, AND DECEMBER. 

360. Andromeda. — Almost directly over head, at 9 
o'clock, on the 15th of November, may be seen the con- 
stellation Andromeda. The figure is that of a woman 
in a sitting posture, with her head to the southwest. 
Andromeda may be known by three stars of the second 
magnitude, situated about 12° apart, nearly in a straight 
line, and extending from east to west. The middle star 
of the three is situated in her girdle, and is called 
Mirach. The one west of Mirach is Alpherat, in the 
head of Andromeda; and the eastern one, called Al- 
maak, is in her left foot. The star in her head is in the 
equinoctial colure. The three largest stars in this con- 
stellation are of the second magnitude. Near Mirach, 
are two stars of the third and fourth magnitudes, and the 
three in a row constitute the girdle. 

This constellation embraces 66' stars, of which three are of the 2d magnitude, two of 
the 3d, and the rest small. About 2° from v, at the northwestern extremity of the 
girdle, is a remarkable cluster or nebula of very minute stars, and the only one of the 
kind which is ever visible to the naked eye. It resembles two cones of light, joined at 
their base, about 1° in length, and £° in breadth. 

361. Pegasus (the Flying Horse). — The figure is the 
head and fore parts of a horse, with wings. The three 
principal stars are of the 2d magnitude — viz., Algenib, 
about 15° south of Alpherat, in Andromeda ; Marlcab, 
about 18° west of Algenib ; and Sheat, 15° south of 
Markab. These three, with Alpherat in Andromeda, 
form what is called the Square of Pegasus. The head 
of the figure is to the southwest, almost in a line with 
Alpherat and Markab, and about 20° from the latter. 

360. Constellations on the meridian, in what months taken up ? An- 
dromeda — where situated ? Figure ? Position ? How known ? Name 
principal stars. (How many stars in constellation? What cluster, and 
where ?) 

361. Figure of Pegasus? Principal stars? How situated? Forming 
what ? How the horse situated ? His head where ? 



DESCRIPTION OF THE CONSTELLATIONS. 175 



362. Pisces (the Fishes) consists of two fishes, distin- 
guished as the northern and western, connected by an 
irregular line of stars. 

The Western Fish is situated directly south of the 
square of Pegasus — is about 20° long, with its head to 
the west. It includes a number of small stars, just 
south of Pegasus. 

The Northern Fish is about the same size, with its 
head near Mirach in Andromeda, and its body extending 
to the north. This, also, includes small stars only, and 
is by no' means conspicuous. 

The flexure or ribbon, uniting the tails of the northern 
and western fishes, extends eastward from the latter, from 
star to star, till it comes opposite the former, when it 
turns to the north, taking several small stars in its way, 
till it joins the northern fish. 

363. Aquarius (the Water-bearer) is represented by 
the figure of a man in a reclining posture, with his head 
to the northwest. Its four largest stars are of the third 
magnitude. It is situated directly south of the head of 
Pegasus, and from 5° to 30° north of a star of the first 
magnitude, in the southern fish. Three of the principal 
stars of Aquarius are near each other in the water-pot 
which he holds in his right hand. 

364. Pisces Australis (the Southern Fish) is situated 
directly south of Aquarius. Its largest star is Fomal- 
haut, of the 1st magnitude, which constitutes the eye of 
the fish. The body extends westy/ard about 20°. 

365. Grus (the Crane) is situated directly south of the 
southern fish, with its head to the north. It is composed 
of a few stars only, of the fourth magnitude. As it is 
45° south -of the equinoctial, it appears low down in the 
south to persons situated in the Middle or Eastern States. 

366. The Phoenix is about 25° east of the Crane. It 

362. Describe Pisces. The Western Fish ? The Northern ? Flexure ? 

363. Figure of 'Aquarius ? Largest stars ? Situation and extent ? Fur- 
ther description. 

364. Pisces Australis — largest star ? Situation of figure ? 

365. Grus — how situated ? Where? Composition? 

366. Situation of the Phoenix f Principal stars ? 



176 ASTRONOMY 



has two stars of the 2d magnitude, about 12° apart east 
and west. The most western of these, in the neck of the 
bird, is about 25° southeast of Fomalhaut, in the South- 
ern Fish. The other stars of the figure are of the 3d 
and 4th magnitudes. 

367. Cassiopeia (the Queen). — About 30° northeast of 
Andromeda is Cassiopeia. The figure is that of a woman 
sitting in a chair, with her head from the pole, and her 
body in the Milky Way. Its four largest stars are of 
the 3d magnitude. 

368. Perseus (the King). — Directly north of the 
" seven stars," and east of Andromeda, is Perseus. The 
figure is that of a man with a sword in his right hand, 
and the head of Medusa in his left. Algol, a star of the 
2d magnitude, is about 18° from the Pleiades (or seven 
stars), in the head of Medusa ; and 90° northeast of Al- 
gol is Algenib, of the same magnitude, in the back of 
Perseus. It embraces four other stars of the third mag- 
nitude, besides many smaller. 

369. Musca (the Fly) is about 12° south of Medusa's 
head. It is a very small constellation, embracing one 
star of the 2d magnitude, two of the 3d, and a few 
smaller. 

370. The Triangles include a few small stars, about 
half-way between Musca in the southeast, and Mirach in 
Andromeda in the northwest. Its two principal stars 
are of the 3d magnitude. 

371. Aries (the Ram). — The head of Aries is about 
10° south of the Triangles. It may be known by two 
stars about 4° apart, of the 3d and 4th magnitudes. The 
most northeasterly of the two is the brightest, and is 
called Arietis. The back of the figure is to the north, 
and the body extends eastward almost to the Pleiades. 

367. Where is Cassiopeia? Figure ? Situation ? Largest stars ? 

368. Perseus — figure ? Two principal stars ? Names ? Situation ? Mag- 
nitude ? 

369. Where is Musca ? Size ? Composition ? 

370. The Triangles — where ? Principal stars ? 

371. Where is Aries? How known? Which of two principal stars 
brightest ? Name ? How figure situated ? Extent ? 



DESCRIPTION OF THE CONSTELLATIONS. 177 



372. Cetus (the Whale). — Directly southeast of Ari- 
etis, and about 25° distant, is Menkar, a star of the 2d 
magnitude, in the mouth of Cetus. This is the largest 
constellation in the heavens. It is situated below or 
south of Aries. It is represented with its head to the 
east, and extends 50° east and west, with an average 
breadth of 20°. The head of Cetus may be known by 
five remarkable stars, 4° and 5° apart, and so situated as 
to form a regular pentagon, or five-sided figure. About 
40° southwest of Menkar, is another star in the body of 
the figure, near which are four small stars nearly in a 
row, and close together, running east and west. 

Passing eastward, we next take the constellations that 
are on the meridian in 

JANUARY, FEBRUARY, AND MARCH. 

373. Taurus (the Bull) will be readily found by the 
seven stars or Pleiades, which lie in his neck. The 
largest star in Taurus is Aldebaran, in the Bull's eye, a 
star of the first magnitude, of a reddish color, somewhat 
resembling the planet Mars. Aldebaran, and four other 
stars in the face of Taurus, compose the Hyades. They 
are so placed as to form the letter V. 

374. Orion lies southeast of Taurus, and is one of the 
most conspicuous and beautiful of the constellations. The 
figure is that of a man in the act of assaulting the bull, 
with a sword in his belt, and a club in his right hand. 
It contains two stars of the first magnitude, four of the 
second, three of the third, and fifteen of the fourth. Be- 
telguese forms the right, and Bellatrix the left shoulder. 
A loose cluster of small stars forms the head. Three 
small stars, forming a straight line about 3° in length, 
constitute the belt, called by Job " the bands of Orion" 
They are sometimes called the Three Kings, because 
they point out the Hyades and Pleiades on the one hand, 

372. Cetus — what star pointed out? Size of constellation? Situation? 
Extent? How know its head f What other star pointed out ? What con- 
stellations next described in order ? 

873. Taurus — how found ? Largest star? Hyades? 

374. Orion— situation ? Character ? Figure ? Composition ? 



178 ASTRONOMY, 



and Sirius on the other. A row of very small stars runs 
down from the belt, forming the sword. These, with the 
stars of the belt, are sometimes called the Ell and 
Yard. Mintika, the northernmost star in the belt, is 
less than £° south of the equinoctial. Bigel, a bright 
star of the first magnitude, is in the left foot, 15° south 
of Bellatrix ; and Saiph, of the third magnitude, is situ- 
ated in the right knee, 8^° east of Rigel. 

375. Lepus (the Hare) is directly south of and near 
Orion. It may be known by four stars of the third mag- 
nitude, in the form of an irregular square. Zeta, of the 
fourth magnitude, is the first star, situated in the back, 
and about 5° south of Saiph in Orion. About the same 
distance below Zeta are the four principal stars, in the 
legs and feet. 

376. Columba Noachi (Noah's Dove) lies about 16° 
south of Lepus. It contains but four stars, of which 
Phaet is the brightest. It lies on the right, a little 
higher than Beta, the next brightest. This last may be 
known by a small star just east of it. 

377. Eridanus (the River Po) is a large and irregular 
constellation, very difficult to trace. It is 130° in length, 
and is divided into the northern and southern streams. 
The former lies between Orion and Cetus, commencing 
near Eigel in the foot of Orion, and flowing out westerly 
in a serpentine course, near 40^, to the Whale. 

378. Canis Major (the Greater Dog) lies southeast of 
Orion, and may be readily found by the brilliancy of its 
principal star, Sirius. This is the largest of the fixed 
stars, and is supposed to be the nearest to the solar sys- 
tem. 

.379. Argo Navis (the Ship Argo) is a large and 
splendid constellation southeast of Sirius, but so low down 
in the south that but little of it can be seen in the United 



375. Where is Lepus ? How known ? Describe. 

376. Columba JVoachi — situation? Composition? 

377. Describe Eridanus. Length ? Division ? Situation ? 



378. W r herc is Canis Major situated ? How found i What of Sirius? 
JYavis. Where situated ? Principal sta 



379. Describe Argo JYavis, Where situated ? Principal stars, and where ? 



DESCRIPTION OF THE CONSTELLATIONS. 179 



States. It lies southeast of Canis Major, and may be 
known by the stars in the prow of the ship. Markeb, of 
the fourth magnitude, is 16° southeast of Sirius. JVaos 
and y, still further south, are of the second magnitude, 
and Canopus and Miaplacidus of the first. 

380. Canis Minor (the Lesser Dog) is situated about 
25° northeast of Sirius, and between Canis Major and 
Cancer. It is a small constellation, having one star, 
Procyon, of the 1st magnitude, and Gomelza, of the 2d. 

381. Monoceros (the Unicorn). — A little more than 
half way from Procyon to Betelguese in Orion, are three 
stars in a row, about 4° apart, and of the 4th magnitude. 
They extend from northeast to southwest, and constitute 
the face of Monoceros. His head is to the west, with 
Canis Minor on his back, and his hind feet about 25° 
southeast of Procyon. It is a large constellation, with 
but few stars, and those mostly small. 

382. Hydra (the Water Serpent).— About 20° east of 
Procyon are four stars of the fourth magnitude, situated 
about 4° apart, and so as to form a diamond; the longer 
axis running east and west. These constitute the head 
of Hydra, which points to the west. The figure extends 
to the south and east more than 100°, taking in an ir- 
regular line of stars of the 3d and 4th magnitudes. The 
largest star is about 15° southeast of the head. It is of 
the 2d magnitude, and is called Alpliard. 

383. Cancer (the Crab) is the least remarkable of the 
zodiacal constellations. It is situated about 15° north of 
the diamond in Hydra. It has no stars larger than the 
3d magnitude, and is distinguished for a group of small 
stars called the Nebula of Cancer, which is often mis- 
taken for a comet. A common telescope resolves this 
nebula into a beautiful assemblage of bright stars. 

384. Gemini (the Twins) may be known by two bright 

880. Canis Minor — where ? Describe. 

881. Where is Monoceros? How situated ? Composed? Character? 

882. Where is the head of Hydra f How formed ? Extent and position ? 
Largest star? 

383, Describe Cancer, Situation? Composition? For what distin- 
guished ? 



180 ASTRONOMY. 



stars of the 2d magnitude — one in the head of each 
figure. They are about 5° apart ; the northeasterly one, 
and the brightest of the two, being about 25° due north 
of Procyon. This is Pollux / and the other one is called 
Castor. The bodies ' of the Twins extend from Castor 
and Pollux about 15° to the southwest, or toward Betel- 
guese, in the right shoulder of Orion. 

" This constellation," says Dr. Adam Clark, " was deemed propitious to mariners ;" 
and on this account, the ship in which St. Paul sailed from Alexandria (Acts xxviii. 11) 
had the sign of Castor and Pollux. 

385. Herschel's Telescope covers two stars of the 5th 
magnitude, near each other, and about 10° north of Cas- 
tor ; and one other star of the same magnitude, about 
10° northwest of the two first named. It is a small affair 
to immortalize Herschel's grand telescope. 

386. The Lynx is situated between Gemini and Can- 
cer on the south, and the Pole in the north, the head 
being to the northwest. It has no stars larger than the 
4th magnitude, and these are in two pairs — the first 15° 
northeast of Cancer, and the other 30° north of it. It is 
a loose and tame constellation, with nothing striking or 
peculiar by which it may be identified. 

387. Camelopardalis (the Camelopard) extends from 
Perseus to the Pole. This, too, is a tame and uninterest- 
ing constellation, with but few stars in it, and those of 
the 4th magnitude, or less. The hind feet of the figure 
touch the Milky Way, and the head is composed of two 
stars of the 5th magnitude, 5° and 10° from the Pole 
star, toward the " dipper" in the Great Bear. 

"We now pass eastward to constellations that are on the 
meridian in 

APRIL, MAY, AND JUNE. 

388. Ursa Major (the Great Bear) is one of the most 
conspicuous in the northern heavens. It may be known 

384. Gemini — how known ? Names and situation of principal stars ? Of 
figures? (Note.) 

385. HersclieVs Telescope — where? Character? 

886. Situation of the Lynx ? Position ? Character ? 

387. Position of Camelopardalis ? Extent ? Character ? Where the feet ? 
The head, and how composed? What range of constellations next de- 
scribed ? 



DESCRIPTION OF THE CONSTELLATIONS. 181 



by the figure of a larger dipper, which constitutes the 
hinder part of the animal. This dipper is composed of 
seven stars. The first, in the end of the handle, is called 
Benetnash, and is of the 2d magnitude. The next is 
Mizar, known by a minute star almost touching it, called 
Alcor. Mizar is a double star. The third in the handle 
is Alioth. The first star in the bowl of the dipper, at 
the junction of the handle, is Megrez. Passing to the 
bottom of the dipper, we find Phad and Merak, while 
Dub he forms the rim opposite the handle. Merak and 
Dubheare called the Pointers, because they always point 
toward the Pole star. The head of the Great Bear lies 
far to the west of the Pointers (apparently east when 
seen below the Pole), and is composed of numerous small 
stars ; while the feet are severally composed of two small 
stars, very near to each other. Megrez in Ursa Major, 
and Coph in Cassiopeia, are almost exactly opposite each 
other, on different sides of the Pole star, and about 
equally distant from it. They are both in the equinoc- 
tial colure. 

389. Leo (the Lion). — About 55° southwest of the 
Pointers is Pegulus, a star of the 1st magnitude, in the 
breast of Leo. This star is situated directly in the 
ecliptic. The head of the figure is to the west, the back 
being to the south. North of Regulus are several bright 
stars, in the form of a sickle, of which Regulus is the 
handle. Denebola is a bright star of the 2d magnitude, 
in the Lion's tail, about 25° northeast of Regulus, and 
35° west of Arcturus. 

390. Leo Minor (the Lesser Lion) is a small cluster 
of stars, of which one is of the 3d, and others of the 5th 
magnitude, about half way between Regulus and the 
Pointers. The head of the figure is northwest, and the 
principal stars form the body in the east, and the fore 
paws, which are extended to the west. 

388. Describe Ursa Major. How known? Names of principal stars? 
Which are the Pointers ? What said of Megrez and Coph ? 

389. Where is Regulus? In what constellation ? How situated? Magni- 
tude ? How Leo placed? Where is the sickle? How constituted? Where 
is Denebola ? 

390. Describe Leo Minor. Where and how situated ? 

16 



182 ASTRONOMY. 



391. Coma Berenices (Berenices' Hair) is a beautiful 
cluster of small stars, about 20° northeast of Denabola, 
and half way from Leo Minor to Arcturus. It has but 
one star as large as the 4th magnitude. 

392. Cor Coroli (Charles's Heart) is a bright star of 
the 3d magnitude, about 12° north of Coma Berenices. 
The figure includes several other stars, east and west, of 
the 5th magnitude. 

393. Bootes (the Bear-driver) is directly east of Coma 
Berenices. The figure is that of a man, with his head 
toward the Pole, and Arcturus, a star of the 1st magni- 
tude, in the left knee. The other stars are of the 3d and 
4th magnitudes. Three small stars, forming a triangle, 
and situated 15° northeast of Arcturus, mark the right 
hand of the figure ; while two stars of the 3d and 4th 
magnitudes, and still further north, mark his shoulders. 
The head is marked by Nekkar, another star of the 3d 
magnitude. 

394. Virgo (the Yirgin) lies directly south of Coma 
Berenices and Bootes. The figure is that of a woman 
with wings, with her head to the west, near Denabola in 
Leo ; and her feet about 40° to the east. S-pica, the prin- 
cipal star, is of the 1st magnitude, about 35° southwest 
of Arcturus. 

395. Crater (the Cup) is composed of six small stars 
30° west of Spica. The largest is of the 4th magnitude. 

396. Corvis (the Crow) is still nearer, being only 15° 
southwest of Spica. It has two stars of the 3d magni- 
tude, and three of the 4th. 

397. Libra (the Balance) is about 25° east of Spica. 
It has two stars of the 2d magnitude, about 10° apart, 
which, with two others of the 3d magnitude southeast of 

S91. Coma Berenices — character? Situation? 

392. Cor Coroli — principal star ? Situation ? 

393. Where is Bootes ? Figure ? Position ? Principal stars ? 

394. Where is Virgo situated ? Figure ? Position ? Principal stars ? 

395. Crater — how situated ? Largest star ? 

396. Corvis — where ? Composition ? 

397. Where is Libra ? Composition ? 



DESCRIPTION OF THE CONSTELLATIONS. 183 



them, form a small quadrilateral figure. Its few remain- 
ing stars are at the east, and of the 4th magnitude. 

39S. Centaurus (the Centaur) is a fine compact con- 
stellation about 30° south or southeast of Spica. It has 
nine stars of the 3d magnitude, mostly in the head of 
the figure. It is too low in the south to be visible in the 
United States, except when near the meridian. 

JULY, AUGUST, AND SEPTEMBER. 

399. Ursa Minor (the Lesser Bear) is composed of a 
few stars near the north pole of the heavens, and mostly 
of the 3d and 4th magnitudes. The back of the figure 
is toward the pole, with its head to the west. The Pole 
star, of the 2d magnitude, is in the extremity of its tail. 

' 400. Draco (the Dragon) is an irregular serpentine con- 
stellation, embracing a large circuit in the polar regions. 
He winds round between the Great and Little Bear, and, 
commencing with the tail, between the Pointers and Pole 
star, is easily traced, by a succession of bright stars 
extending from west to east. Passing south of Ursa 
Minor, around nearly to Cephens, it returns westward, 
and terminates in four stars, w^hich form the head, near 
the foot of Hercules. These four stars are 3°, 4°, and 5° 
apart, so situated as to form an irregular square ; the two 
upper ones, Etamis and Rastaban, being the brightest, 
and both of the 2d magnitude. 

401. Hercules (the Giant) is a large, but not very 
striking or conspicuous constellation. The figure is that 
of a giant, with a large club in his right hand, and a 
hydra in his left. The head of the figure is to the south, 
and the whole is composed of stars from the 2d to the 4th 
magnitude. This constellation is thickly set with stars, 
the largest of which is called RasalgetM, in the head 



398. Describe Centaurus. Position? Composition? 

399. Ursa Minor — position ? Principal star ? 

400. Draco — position ? How traced ? W T here liead ? How composed ? 
"Form what ? 

401. Hercules— figure ? Situation? Composition? Principal star? Num- 
ber of stars ? 



184 ASTRONOMY. 



of the figure, and is of the 2d magnitude. It has nine 
stars of the 3d magnitude, and 19 of the 4th. 

402. Coroni Borealis (the Northern Crown) is about 
15° west of the middle of Hercules. Its principal star 
is Alphacca, a bright star of the 2d magnitude, about 
20° northeast of Arcturus. About the same distance, 
directly east of Arcturus, is a small group of stars, which 
constitute the head of the Serpent. 

403. Scorpio (the Scorpion) is one of the most interest- 
ing and splendid of the constellations. It is situated 
about 45° east of Spica, adjoining Libra. The head of 
the figure is composed of five stars — one of the 2d, and 
the others of the 3d magnitude — forming an arc of a cir- 
cle convex to the west. The largest of these five stars is 
in the ecliptic, and is called Graffias. About 9° south- 
east of Graffias is Antares, a star of the 1st magnitude, 
in the body of the figure, and of a reddish color. A 
number of bright stars of the 4th magnitude extend to 
the southeast into the Milky Way, and then curve around 
to the east and north, forming the tail of Scorpio. 

404. Lepus (the Wolf) consists of a small group of 
stars, about 15° southwest of Antares. The head of the 
figure is to the north. 

405. Serpentarius (the Serpent-bearer) is a large but 
uninteresting constellation, between Scorpio on the south, 
and Hercules on the north. The figure is that of a man 
grasping a serpent, the head of which has already been 
described (402). The folds of the serpent may be traced 
by a succession of bright stars extending for some dis- 
tance to the east. The principal star in Serpentarius is of 
the 2d magnitude, and is called lias Alhague. It is 
situated in the head of the figure, and within 5° of Ea- 
salgethi, in the head of Hercules. The feet of the figure 

402. Coroni Borealis— location ? Principal star? What other group of 
stars mentioned ? 

403. Describe Scorpio. Situation? Composition? Largest star in head ? 
W T hat other large star? Position and composition of tail ? 

404. Lepus — composition? Position? 

405. Serpenta rius— situation ? Figure? Principal star? Situation? 



DESCRIPTION OF THE CONSTELLATIONS. 185 



rost upon Scorpio, and the right shoulder touches the 
Milky Way. 

406. Lyra (the Harp) is a small constellation 15° east 
of Hercules. Its principal star is Vega, of the 1st mag- 
nitude, one of the brightest stars in the northern hemi- 
sphere. It has two stars of the 3d magnitude, and sev- 
eral others of the 4th. 

407. Cygnus (the Swan) is situated directly east of 
Lyra. Three bright stars, w T hich lie along the Milky 
Way, form the body and neck of the Swan, running 
northeast and southwest ; and two others, at right angles, 
in a line with the middle one of the three, constitute the 
icings. These five stars form a large cross. Arided, in 
the body of the Swan, is a star of the 1st magnitude, 
and the remaining ones of the constellation are of the 
3d and 4th. 

408. The Fox and Goose is located just south of Cyg- 
nus, w r ith the head to the west. It is a small constella- 
tion ; the two principal stars of which, of the 2d magni- 
tude, form the head of the Fox. Most of the figure is 
in the Milky Way. 

409. Aquila (the Eagle) is still south of Cygnus and 
the Fox. It is conspicuous for three bright stars in its 
neck, of which the central one, Altair, is a brilliant 
white star of the first magnitude, just east of the Galaxy. 

410. Delphinus (the Dolphin) is a beautiful little clus- 
ter of stars, 15° northeast of the Eagle. It may be 
known by four principal stars in the head, of the 3d 
magnitude, arranged in the figure of a diamond, and 
pointing northeast and southwest. A star of the same 
magnitude, about 5° south, makes the tail. 

411. Antonius lies directly south of Aquila, his head 
being near Altair, and the body and feet to the south- 
west. Two stars of the 3d magnitude constitute the right 

406. Lyra — situation ? Principal star ? What others ? 

407. Cygnus — situation ? Composition ? 

408. Fox and Goose — location ? Position of figure ? 

409. Aquila — where ? For what conspicuous \ 

410. Describe Delpldnus. How known ? 

411. Antonius — situation? How placed ? Composition? 

16* 



186 ASTR0N0MT. 



arm, and several smaller ones make the bow and arrows 
held in his hand. 

412. Sagittarius (the Archer) lies next to Scorpio, and 
may be known by three stars in the Galaxy, arranged in 
a curve, to represent the how of the archer. The central 
star is the brightest, and has a bright star directly west 
of it, forming the head of the arrow. The head and 
chest of Sagittarius are just east of the Milky Way, be- 
tween the tail of Scorpio and the head of Capricornus. 

413. Capricornus (the Goat) is situated about 20° 
northeast of Sagittarius. The head of the figure is to 
the west, and is composed of two bright stars, of the 3d 
magnitude, and about 4° apart. There is a smaller star 
between them, and several still smaller close around 
them. 

414. Crux (the Cross) is a brilliant little constellation, 
but too far south to be visible to us at the north. It con- 
sists of four principal stars — namely, one of the 1st, two 
of the 2d, and one of the 3d magnitude. 

Besides these, there are several fine constellations about the south pole of the heav- 
ens, as the Altar, the Peacock, Charles's Oak, &c. ; but as they cannot be traced from 
the latitudes in which this book will be used, it is thought not important to describe 
them. 



412. Sagittarius — where ? How known ? 

413. Capricornus — where? Position of figure ? Composition? 

414. Crux — describe. Composition ? (What said of south circumpolar 
constellations ? Names ? Why not described ?) 



DOUBLE STARS. 187 



CHAPTER III. 

DOUBLE, VARIABLE, AND TEMPORARY STARS, BINARY SYSTEMS, ETC. 

415. Many of the stars which, to the naked eye, ap- 
pear single, are found, when examined by the aid of a 
telescope, to consist of two or more stars, in a state of 
near proximity to each other. These are called Double 
Stars. When three or more stars are found thus closely 
connected, they are called Triple or Multiple Stars. 

416. Double and triple stars are supposed to be consti- 
tuted in two ways — first, by actual contiguity ; and 
secondly, where they are only near the same line of 
vision, one of the component stars being far beyond the 
other. In the former case, they are said to he physically 
double, from the belief that they are bound together by 
attraction, and that one revolves around the other ; while 
in the latter case, they are considered as only optically 
double. 

BTAES OPTICALLY DOUBLE. 

Apparent positions. True positions. 

_.. - * 

IT"—"" r * 

A B 

Here the observer on the left sees a large and small star at A apparently near toge- 
ther — the lowest star being much the smallest. But instead of their being* situated as 
they appear to be, with respect to each other, the true position of the smaller star may 
be at B instead of A ; and the difference in their apparent magnitudes may be wholly 
owing to the greater distance of the lower star. 

Upon this subject Dr. Herschel remarks, that this nearness of the stars to each other, 
in certain cases, might be attributed to some accidental cause, did it occur only in a few 
instances ; but the frequency of this companionship, the extreme closeness, and, in 
many cases, the near equality of the stars so conjoined, would alone lead to a strong 
suspicion of a more near and intimate relation than mere casual juxtaposition. 

415. What said of double, triple, and multiple stars ? 

416". How are they supposed to be constituted ? How distinguished ? 
(Illustrate by diagram. Kemark of Dr. Herschel? How many specimens 
of double stars given ?) 



188 



ASTRONOMY, 



The following will convey to the student an idea of the telescopic appearance of some 
of the double stars : 

SPECIMENS OF DOUBLE STARS. 




417. A is a double star in Ursa Minor, commonly 
known as the Pole star. It consists of a star of the 2d, 
and another of the 9th magnitude, situated about 18" 
apart, or about four times the diameter of the larger 
star. They are both of a silvery white. It requires a 
pretty good telescope to show r this star double ; hence it 
is considered a very good test object by which to ascer- 
tain the qualities of a telescope, especially of the low- 
priced refractors. 

The writer has often seen the companion of the Pole star distinctly, with a six-inch 
refracting telescope, manufactured by Mr. Henry Fitz, New York. 

418. B is a view of the double star Castor, in the 
Twins. The stars are of a greenish w T hite, of the 3d and 
4th magnitudes, and about 5", or two diameters of the 
principal star, apart. This also is considered a good 
test object. Through ordinary telescopes, the stars seem 
to be in contact; but with- those of higher power, they 
appear fairly divided. These stars constitute what is 
called a Binary System. 

419. C is a representation of Mizar, the middle star, 
in the tail of the Great Bear. It may be seen double 
with a good spy-glass. The stars are both of a greenish 
white, of the 3d and 4th magnitudes, and about 14' 
apart. Mizar has sometimes been seen without a com- 
panion, and at other times it has been known suddenly 
to appear. The companion is not Alcor, near Mizar, and 
visible to the naked eye, but a telescopic star. 



417. What is Fig. A in the cut ? How composed ? Color ? How seen ? 
(Remark of author in note ?) 

418. Fig. B — color? Magnitudes? Distance apart? Further remark? 

419. Fig. C— how seen ? Color ? Magnitude ? Distance ? Additional 
remarks ? 



BINARY AND OTHER SYSTEMS. 189 



420. D is a view of the double star Mintaka, the mid- 
dle star of the three forming the belt of Orion. The 
component stars are of the 4th and 8th magnitudes — the 
largest of a reddish hue, and the small one white. They 
are about 10" apart, or four times the diameter of the 
largest star. 

421. E is a view of Higel, in the left foot of Orion. 
The components are of the 1st and 9th magnitudes, and 
about 10" apart. Their color is a yellowish white. 

422. F is a view of the bright star Vega, in the Lyre. 
Its companion is a star of the 11th magnitude, situated 
about 40" distant. This is a close test object for an ordi- 
nary telescope. 

423. The number of double stars has been variously 
estimated. Sir William Herschel enumerates upwards 
of 500, the individuals of which are within 30" of each 
other. Professor Struve of Dorpat estimated the num- 
ber at about 3,000 ; and more recent observations fix the 
number at not less than 6,000. The great number of the 
double stars first led astronomers to suspect a physical 
connection by the laws of gravitation, and also a revolu- 
tion of star around star, as the planets revolve around 
the sun. 

BINARY AND OTHER SYSTEMS. 

424. By carefully noting the relative distances and 
angular positions of double and multiple stars, for a se- 
ries of years, it has been found that many of them have 
their periodic revolutions around each other. "Where 
two stars are found in a state of revolution about a com- 
mon center, they constitute what is called a Binary Sys- 
tem. These, it must be remembered, are the double and 
multiple stars, w T hich appear single to the naked eye. 
Sir W. Herschel noticed about 50 instances of changes 
in the angular position of double stars ; and the revolu- 

420. Fig. D — describe. Magnitude ? Color ? Distance ? 

421. Fig. E — place? Components? Distance? Color? 

422. Fig. F — companion? 

423. Number of double stars ? Led to what? 

424. Motions of double stars ? What are binary system* ? 



190 ASTRONOMY. 



tion of some eighteen of these is considered certain. 
Their periods vary from 40 to 1,200 years. 

425. The star numbered 70 in the Serpent-bearer is a 
binary system. The periodic time of the revolving star 
is about 93 years. In the course of its revolution, the 
two stars sometimes appear separated, sometimes very 
near together, and at other times as one star. They are 
of the 5th and 6th magnitudes, and of a yellowish hue. 

426. The star g, in the left hind paw of Ursa Major, 
is one of these stellar systems. The revolution of its 
component stars began to be noticed in 1781 ; since 
which time they have made one complete revolution, 
and are now (1853) some fourteen years on the second. 
Of course, then, their periodic time is about 58 years. 
Their angular motion is about 6° 24' per year. 

Dr. Dick supposes these stars to be some 200,000,000,000 miles apart; and upon th< 
supposition that the smaller revolves around the latter, computes its velocity to be nol 
less than 2,471,000 miles every hour. This would be 85 times the velocity of Jupiter 
and 23 times the velocity of Mercury — the swiftest planet in the solar system. 

427. The star y in Yirgo is another of these systems. 
It has been known as a double star for at least 130 years. 
The two stars are both of the 3d magnitude, and of a 
yellowish color. The late E. P. Mason, of Yale College, 
estimated its period at 171 years. More recent observa- 
tions and estimates by Madler give a period of 145 
years. 

428. "To some minds, not accustomed to deep reflec- 
tion," says Dr. Dick, " it may appear a very trivial fact 
to behold a small and scarcely distinguishable point of 
light immediately adjacent to a larger star, and to be in- 
formed that this lucid point revolves around its larger 
attendant ; but this phenomenon, minute and trivial as 
it may at first sight appear, proclaims the astonishing 
fact, that suns revolve around suns, and systems around 
sy steins. This is a comparatively new idea, derived from 
our late sidereal investigations, and forms one of the 

425. Describe 70 Opliiuchi ? 

426. What specimen described ? Period? Yearly angular motion ? (Dr. 
Dick's remark ?) 

427. What other binary system ? How long known ? Components ? 
Period ? 

428. Quotation from Dr. Dick. 



BINARY AND OTHER SYSTEMS. 191 



most sublime conceptions which the modern discoveries 
of astronomy have imparted. 

429. "It undoubtedly conveys a very sublime idea, 
to contemplate such a globe as the planet Jupiter — a 
body thirteen hundred times larger than the earth — re- 
volving around the sun, at the rate of twenty-nine thou- 
sand miles every hour ; and the planet Saturn, with its 
rings and moons, revolving in a similar manner round 
this central orb, in an orbit five thousand six hundred 
and ninety millions of miles in circumference. But how 
much more august and overpowering the conception of a 
sun revolving around another sun — of a sun encircled 
with a retinue of huge planetary bodies, all in rapid mo- 
tion, revolving round a distant sun, over a circumference 
a hundred times larger than what has been now stated, 
and with a velocity perhaps a hundred times greater than 
that of either Jupiter or Saturn, and carrying all its 
planets, satellites, comets, or other globes, along with it in 
its swift career ! Such a sun, too, may as far exceed 
these planets in size as our sun transcends in magnitude 
either this earth or the planet Venus ; the bulk of any 
one of which scarcely amounts to the thirteen-hundred- 
thousandth part of the solar orb which enlightens our 
clay. 

430. " The further we advance in our explorations of 
the distant regions of space, and the more minute and 
specific our investigations are, the more august and as- 
tonishing are the scenes which open to our view, and the 
more elevated do our conceptions become of the gran- 
deur of that Almighty Being who ' marshalled all the 
starry hosts,' and of the multiplicity and variety of ar- 
rangements he has introduced into his vast creation. 
And this consideration ought to serve as an argument 
to every rational being, both in a scientific and a reli- 
gious point of view, to stimulate him to a study of the 
operations of the Most High, who is ' wonderful in coun- 
sel, and excellent in working, 5 and whose works in every 

429. What further remarks ? 

430. Continue quotation. (What table ? Note ?) 



192 



ASTRONOMY 



part of his dominions adumbrate the glory of his perfec- 
tions, and proclaim the depths of his wisdom, and the 
greatness of his power." 

The following table shows the periodic times of the 
principal binary systems, so far as known : 



BINARY SYSTEMS. 



Names. 


Period in 


Names. 


Period in 




years. 




years. 


y\ Coronse .... 


43-40 


w Leonis 


82-533 


£ Cancri .... 


55-00 


1 Bootes 


117-140 


1 Ursae Majoris 


58-26 


a Hercules . . . 


31-468 


70 Ophiuchi . . . 


93-00 


b Ursse Majoris 


58-262 


61 Cygni 


452-00 


c " u 


61-464 


y Virginis . . . 


145-00 


p Ophiuchi . . . 


73-862 


Castor 


286-00 


b " ... 


80-340 


c Coronse .... 


145-00 


c " ... 


92-870 


y Leonis 


1200-00 


f3 Coronse .... 


608-450 



The student should here be reminded that these are not systems of planets revolving 
around suns, but of sun revolving around sun; and that their component stars may 
not only be as far apart as our sun and Sirius, but that they are probably each the center 
of his own planetary system, like that which revolves around our central orb. 

431. Besides the revolutions of these double stars 
around each other, they are found to have a proper mo- 
tion together in space, like that which our sun has around 
the great central Sun. Upon this subject Sir John Her- 
schel observes, that these stars not only revolve around 
each other, or about their common center of gravity, but 
that they are also transferred, without parting company, 
by a progressive motion common to both, toward some 
determinate region. 

The two stars of 61 Cygni, which are nearly equal, have remained constantly at the 
same, or very nearly the same, distance of 15", for at least 50 years past. Meanwhile, 
they have shifted their local situation in the heavens, in this interval of time, through 
no less than 4' 23" — the annual proper motion of each star being 5".3; by which quan-. 
tity (exceeding a third of their interval) this system is every year carried bodily along 
in some unknown path, by a motion which, for many centuries, must be regarded as 
uniform and rectilinear. Among stars not double, and no way differing from the rest in 
any other obvious particular, ^ Cassiopeia is to be remarked as having the greatest 
proper motion of any yet ascertained, amounting to 3".74 of annual displacement. 



431. What other motion of the stars ? Dr. Herschel ? (Specimen in note ? 
Motions ? What star named as having the greatest proper motion of any yet 
known?) 



STAES OF VARIOUS COLOKS. 193 



432. But though motions which require whole centu- 
ries to accumulate before they produce changes of ar- 
rangement, such as the naked eye can detect, are quite 
sufficient to destroy that idea of mathematical fixity 
which precludes speculation, yet are they too trifling, so 
far as practical applications go, to induce a change of 
language, and lead us to speak of the stars, in common 
parlance, as otherwise than fixed. 

433. Most of the double, triple, and multiple stars are 
of various colors, beautifully contrasting with each other. 

Other suns, perhaps, 



"With their attendant moons 
Communicating male and female light, 
(Which two great sexes animate the world,) 
Stored in each orb, perhaps, with some that live." 

It is probable, however, that, in most cases, this variety 
of colors is merely complimentary, in accordance with 
that general law of optics which provides that when the 
retina is under the influence of excitement, by any bright 
colored lights, feebler lights, which, seen alone, would 
produce no sensation but of whiteness, shall for the time 
appear colored with the tint complimentary to that of 
the brighter. Thus, if a yellow color predominate in 
the light of the brighter star, that of the less bright one 
in the same field of view will appear blue ; while, if the 
tint of the brighter star verge to crimson, that of the 
other will exhibit a tendency to green, or even appear as 
a vivid green, under favorable circumstances. 

434. This first law of contrast is beautifully exhibited 
by i Cancri — the latter by 7 Andromedse ; both fine 
double stars. If, however, the colored star be much 
the less bright of the two, it will not materially affect 
the other. Thus, for instance, r\ Cassiopeise exhibits the 
beautiful combination of a large white star, and a small 
one of a rich ruddy purple. 

It is by no means, however, intended to say, that in all such cases one of the colors 
is a mere effect of contrast; and it may he easier suggested in words than conceived in 



432. Why called " fixed stars," if in motion ? 

433. What said of the color of double stars ? Quotation from Milton i 
Cause of this variety of colors? 

434. Specimens 01 complimentary colors ? (Are they all complimentary ?) 

17 



194 ASTRONOMY. 



imagination what variety of illumination tioo suns — a red and a green, or a yellow and 
a blue one — must afford a planet circulating about either, and what charming contrasts 
and " grateful vicissitudes'" — a red and a green day, for instance, alternating with a whito 
one and with darkness — might arise from the presence or absence of one or other, or 
both, above the horizon. Insulated stars of a red color, almost as deep as that of blood, 
occur in many parts of the heavens ; but no green or blue star, of any decided hue, has, 
we believe, ever been noticed unassociatcd with a companion brighter than itself. 



VARIABLE OR PERIODICAL STARS. 

435. "Variable stars are those which undergo a regular 
periodical increase and diminution of lustre, amount- 
ing, in some cases, to a complete extinction and revival. 
These variations of brilliancy, to which some of the 
fixed stars are subject, are reckoned among the most 
remarkable of celestial phenomena. Some of them pass 
through their successive changes with great rapidity ; 
while in other cases, their brilliancy is increased or 
diminished gradually for months. The time occupied by 
one of these stars, in passing through all their different 
phases, is called its period. 

436. One of the most remarkable of these variable 
stars is the star Omicron, or Mira in the Whale. Its 
period is about 332 days, during which time it varies 
from a star of the 2d magnitude to complete invisibility. 
Tt appears about twelve times in eleven years — remains 
at its greatest brightness about a fortnight ; being then, 
on some occasions, equal to a large star of the 2d magni- 
tude. It then decreases for about three months, when it 
disappears. In about five months, it becomes visible 
again, and continues to increase during the remaining 
three months of its period. 

Its increase of light is much more rapid than its de- 
crease. It increases from the 6th to the 2d magnitude 
in 40 days, continues thus brilliant 26 days, and then 
fades to the 6th magnitude again in 66 days. Hence it 
is above the 6th magnitude for 132 days, and below 200 
days of its period. 



435. What are variable stars ? How regarded ? What difference ? What 
their period ? 

486. What remarkable sample described ? Period ? Amount of variation ? 
Progress of variation ? 



VARIABLE OR PERIODICAL STARS. 195 



437. Another remarkable periodic star is that called 
Algol, in the constellation Perseus. It is usually visible 
as a star of the 2d magnitude, and such it continues for 
the space of 2 days 14 hours} when it suddenly begins 
to diminish in splendor ; and in about 3 J hours, it is re- 
duced to the 4th magnitude. It then begins again to 
increase, and in 3^ hours more is restored to its usual 
brightness ; going through all its changes in 2 days 20 
hours and 48 minutes, or thereabouts. Through all its 
successive changes, this star shines with a white light, 
while the. color of all the other variable stars is red. 

438. The cause of these periodic variations in the 
brightness of some of the stars is not known. Some 
suppose them to be occasioned by opake bodies revolv- 
ing around them, and cutting off a portion of their light 
from us. Speaking of the sudden obscuration of Algols 
mentioned above, Dr. Herschel remarks that it indicates 
a high degree of activity in regions where, but for such 
evidences, we might conclude all lifeless. 

439. "I am disposed," says Dr. Dick, "to consider it 
as highly probable, that the interposition of the opake 
bodies of large planets revolving around such stars may, 
in some cases, account for the phenomena. ' 

" It is true that the planets connected with the solar system are so small, in comparison 
of the sun, that their interposition between that orb and a spectator, at an immense dis- 
tance, would produce no sensible effect. But we have no reason to conclude that in all 
other systems the planets are formed in the same proportions to their central orbs as 
ours ; but from the variety we perceive in every part of nature, both in heaven and 
earth, we have reason to conclude that every system of the universe is, in some re- 
spects, different from another. There is no improbability in admitting that the planets 
which revolve round some of the stars may be so large as to bear a considerable propor- 
tion (perhaps one-half or one-third) to the diameters of the orbs around which they re- 
volve ; in which case, if the plane of their orbit lay nearly in a line of our own vision, 
they would, in certain parts of their revolutions, interpose between our eye and the 
stars, so as to hide for a time a portion of their surfaces from our view, while in that 
part of their orbits which is next to the earth." 

440. Others, again, are of opinion that those distant 
suns have one luminous and one opake or clouded hemi- 
sphere ; and that their variations may thus result from a 
revolution upon their axes, by which they would present 
us alternately with their full and their diminished luster. 

437. What other specimen ? Usual appearance ? Period ? Peculiar color ? 

438. Cause of these variations ? Supposition ? Dr. Herschel ? 

439. Dr. Dick's opinion? (Reasoning in note ?) 

440. What other hypothesis stated ? ^ 



196 ASTRONOMY. 



Another theory is, that these stars are moving with 
inconceivable velocity in an immense elliptical orbit, 
the longer axis of which is nearly in a direction toward 
the eye, and the shorter axis of which would be imper- 
ceptible from our system. In such case, the star w T ould 
appear alternately to approach and recede ; now looking 
in upon our quarter of the universe for a few days, and 
then rushing back into immensity, to be seen no more 
by human eyes during the lapse of years or of ages. 

441. " "Whatever may be the cause" says Mr. Abbott, 
" the fact of these variations is perfectly established, and 
the contemplation of the stupendous changes which must 
be occurring in those distant orbs overwhelms the mind 
with amazement. "Worlds vastly larger than our sun sud- 
denly appear, and as suddenly disappear. Now they 
blaze forth with most resplendent brilliancy, and again 
they fade away, and often are apparently blotted from 
existence. These w r orlds are unquestionably thronged 
with myriads of inhabitants ; and the phenomenon which 
to us appears but as the w T axing or waning lusier of a 
twinkling star, may, to the dwellers on these orbs, be 
evolutions of grandeur, such as no earthly imagination 
has ever conceived. But these scenes, now veiled from 
human eyes, will doubtless all be revealed, when the 
Christian shall ascend on an angel's wing to the angel's 
home." 

TEMPORARY STARS. 

442. Temporary stars are those which have appeared 
from time to time in different parts of the heavens, blaz- 
ing forth with extraordinary luster, and, after remaining 
for a while apparently immovable, died away, and left 
no traces of their existence behind. Some writers class 
them among the periodical stars, while others notice them 
under the head of " New and Lost Stars." 

A star of this kind, which appeared in the year 125 



441. Remarks of Mr. Abbott? 

442. What are temporary stars? How classed? First noticed? What 
other instance ? 



TEMPORARY STARS. 197 



b. a, led Hipparchus to draw up a catalogue of the stars, 
the earliest on record. In a. d. 389, a similar star ap- 
peared near the largest star in the Eagle, which, after 
remaining for three weeks as bright as Venus, disap- 
peared entirely from view. 

443. On the 11th of November, 1572, Tycho Brahe, a 
celebrated Danish astronomer, was returning, in the 
evening, from his laboratory to his dwelling-house, wheiij 
he was surprised to find a group of country people gazing! 
upon a star which he was sure did not exist half an hourj 
before. It was then as bright as Sirius, and continued 
to increase till it surpassed Jupiter in brightness, and 
was visible at noonday. In December of the same year 
it began to diminish ; and in March, 1574, had entirely 
disappeared. 

This remarkable star was in the constellation Cassio- 
jyeia, about 5° northeast of the star Cajph. The place 
where it once shone is now a dark void ! 

444. This star was observed for about 16 months, and 
during the time of its visibility its color exhibited all the 
different shades of a prodigious flame. " First it was of 
a dazzling white, then of a reddish yellow, and lastly of 
an ashy paleness, in which its light expired." " It is im- 
possible," says Mrs. Sumerville, " to imagine any thing 
more tremendous than a conflagration that could be visi- 
ble at such a distance." 

445. In reference to the same phenomenon, Dr. Dick 
observes, that " the splendor concentrated in that point 
of the heavens where the star appeared must have been, 
in reality, more than equal to the blaze of twelve hundred 
thousand worlds such as ours, were they all collected 
into one mass, and all at once wrapped in flames. Nay, 
it is not improbable, that w r ere a globe as large as would 
fill the whole circumference of the earth's annual orbit 
to be lighted up w T ith a splendor similar to that of the 



443. What other remarkable instance described? By whom? When? 
In what constellation ? Position ? 

444. How long observed ? Appearance ? Mrs. Sumerville ? 

445. Dr. Dick's remarks ? 



198 ASTRONOMY. 



sun, it would scarcely surpass in brilliancy and splendor 
the star to which we refer." 

446. Kev. Prof. Yince, who has been characterized as 
" one of the most learned and pious astronomers of the 
age," advances the opinion, that "the disappearance of 
some stars may be the destruction of that system at the 
time appointed by the Deity for the probation of its in- 
habitants ; and the appearance of new stars may be the 
formation of new systems for new races of beings then 
called into existence to adorn the works of their Creator." 

447. La Place, whose opinion upon such subjects is 
always entitled to consideration, says : " As to these stars 
which suddenly shine forth with a very vivid light, and 
then immediately disappear, it is extremely probable 
that great conflagrations, produced by extraordinary 
causes, take place on their surface. This conjecture is 
confirmed by their change of color, which is analogous 
to that presented to us on the earth by those bodies which 
are set on fire, and then gradually extinguished." 

448. Dr. John Mason Goode, author of the Book of 
Nature, &c, seems to have entertained opinions similar 
to those already expressed. " Worlds and systems of 
worlds," says he, u are not only perpetually creating, but 
also perpetually disappearing. It is an extraordinary 
fact, that within the period of the last century, not less 
than thirteen stars, in different constellations, seem to 
have totally perished, and ten new ones to have been 
created. In many instances, it is unquestionable, that 
the stars themselves, the supposed habitations of other 
kinds or orders of intelligent beings, together with the 
different planets by which it is probable they were sur- 
rounded, have utterly vanished, and the spots they occu- 
pied in the heavens have become blanks. What has 
befallen other systems will assuredly befall our own. Of 
the time and manner we know nothing, but the fact is 
incontrovertible; it is foretold by revelation; it is in- 
scribed in the heavens ; it is felt through the earth. Such 
is the awful and daily text. What, then, ought to be the 
comment ?" 

446. Prof. Vince's remarks ? 447, La Place's ? 448. Dr. Goode's ? 



CLUSTERS OF STAES AND NEBULA. 



199 



CHAPTER IV. 



CLUSTERS OF STARS AND NEBULAE 



TELESCOPIC VIEW OF THE PLEIADES. 



449. In surveying the concave of the heavens in a 
clear night, we observe here and there groups of stars, 
forming bright patches, as 
if drawn together by some 
cause other than casual 
distribution. Such are the 
Pleiades and Hyades in 
Taurus. These are called 
Clusters of Stars. The 
luminous spot called the 
Bee Hive, in Cancer (visi- 
ble to the naked eye), is 
somewhat similar, but less 
definite, and requires a 
moderate telescope to re- 
solve it into stars. In the 
sword-handle of Perseus is 
another such spot or clus- 
ter, which is also visible to 
the naked eye, but which 
requires a rather better telescope to resolve it into dis- 
tinct stars. When fairly in view, however, it is one of 
the most splendid and magnificent spectacles upon which 
the eye can rest. 

" O what a confluence of ethereal fires, 
From worlds unnumber'd down the steep of heaven, 
Stream to a point, and center on my sight." 

450. Many of these faint and compact clusters have 
been mistaken for comets, as through telescopes of mod- 




449. Clusters? Specimens? 

450. What mistake respecting? 
not comets ? 



What like? How known that thev are 



200 ASTKONOMY. 



erate power they appear like such. Messier lias given a 
list of 103 objects of this sort, with which all who search 
for comets ought to be familiar, to avoid being misled by 
their similarity of appearance. That they are not comets, 
is evident from their fixedness in the heavens, and from 
the fact, that when we come to examine them with in- 
struments of great power, they are perceived to consist 
entirely of stars, crowded together so as to exhibit a defi- 
nite outline, and to run up to a blaze of light in the cen 
ter, where their condensation is usually the greatest. 

451. Some of these clusters are of an exceedingly 
rough figure, and convey the idea of a globular space 
filled full of stars, insulated in the heavens, and consti- 
tuting in itself a family or so- 

* 4. ^^~ 4- -<? ~™, 4-"L~ ,.~~4. «^^l ROTTND CLUSTER IN CAPRICORN. 

ciety apart irom the rest, and 
subject only to its own internal 
laws. 

It would be a vain effort to 
attempt to count the stars in 
one of these clusters. They are 
not to be reckoned by hundreds ; 
and on a rough calculation, 
grounded on the apparent inter- 
vals between them at the bor- 
ders, and the angular diameter 

of the whole group, it would appear that many clusters 
of this description must contain, at least, from ten to 
twenty thousand stars, contacted and wedged together 
in a round space, whose angular diameter does not ex- 
ceed eight or ten minutes, or an area equal to a tenth 
part of that covered by the moon. 

452. Some of these clusters have a very irregular out- 
line. These are generally less rich in stars, and especi- 
ally less condensed toward the center. They are also less 
definite in point of outline. In some of them, the stars 
are nearly all of a size ; in others, extremely different. 
It is no uncommon thing to find a veiy red star, much 

451. What said of the form of these clusters ? Stars in each ? Apparent 
diameter ? 

452. What further respecting forms ? Character of irregular clusters ? 




NEBCIN. 



201 



RICH CLUSTER IN BERENICES 1 HAIR. 



brighter than the rest, occupying a conspicuous situation 
in them. 

453. It is by no means improbable that the individual 
stars of these clusters are suns like our own, the centers 
of so many distinct systems, and that their mutual dis- 
tances are equal to those 
which separate our sun from 
the nearest fixed stars. Be- 
sides, the round figure of 
some of these groups seems 
to indicate the existence of 
some general bond of union, 
of the nature of an attractive 
force. 

This is one of the most gorgeous clusters 
in all the heavens. Sir John Herschel pro- 
nounced it the most magnificent object he 
had ever beheld. It is about 6' in diameter, 
and contains a countless throng of stars, 
that scarcely ever fail to elicit a burst of sur- 
prise and astonishment from the beholder ! 

Who can gaze upon such a scene, and not for a time forget earth, in the rapt contempla- 
tion of the distant glory? 

" There's not a scene to mortals given, 
That more divides the soul and clod, 
Than yon proud heraldry of heaven — 
Yon burning blazonry of God. 11 

A similar cluster, though somewhat different in form, may be found between £ and V, 
in Hercules. This, too, is a most magnificent object. Under favorable circumstances, 
it may be seen with the naked eye ; and by the aid of telescopes, it is easily resolved 
into myriads of stars. " It is, indeed, truly glorious, 11 says Smyth, "and enlarges on the 
eye by studious gazing. 11 " Perhaps" says Prof. Nichol, kt no one ever saw it, for the first 
time, through a telescope, without uttering a shout of wonder. 11 




NEBULJ3. 

4:54:. The term Nebalw is applied to those clusters of 
stars that are so distant as to appear only like a faint 
cloud or haze of light. In this sense, some of the clus- 
ters heretofore described maybe classed as nebulae ; and, 
indeed, it may be said of all the different kinds of nebu- 
lae, that it is impossible to say where one species ends, 
and another begins. 



453. What said of individual stars in clusters ? Of round figure of some 
clusters? (What specimen in cut? What said of it ? Angular diameter ? 
Effect of seeing ? Poetry ■ What other similar cluster ? What said of it?) 

454. Whatiire Nebulas? How differ from clusters \ 

9* 



202 ASTRONOMY. 



455. Resolvable NebulcB are those clusters, the light of 
whose individual stars are blended together, when seen 
through a common telescope ; but which, when viewed 
through glasses of sufficient power, can be resolved into 
distinct stars. 

456. Irresolvable Nehulce are those nebulous spots 
which were formerly supposed to consist of vast fields of 
matter in a high state of rarefaction, and not of distinct 
stars. But it is doubtful whether any nebulae exist which 
could not be resolved into stars, had we telescopes of 
sufficient power. 

" About the close of last year," says Dr. Scoresby, in 
1846, " the Earl of Eosse succeeded in getting his great 
telescope into complete operation ; and during the first 
month of his observations on fifty of the unresolvable 
nebulae, he succeeded in ascertaining that 43 of them 
were already resolvable into masses of stars. Thus is 
confirmed the opinion, that we have only to increase the 
power of the instrument to resolve all the nebulae into 
stars, and the grand nebular hypothesis of La Place into 
a splendid astronomical dream." 

a » hr -fcT- t i r> i DOUBLE NEBULA. 

4o7. .Nebulae of almost every 
conceivable shape may be found in 
the heavens. Some are round — 
others elliptical. Some occur sin- 
gly, while others are double, or 
seem to be connected together. 

The specimen here shown is in the Greyhound. The 
two nebulae are elliptical, as shown, and are so united 
as to stand perpendicularly to each other. 

458. Annular Nehulce are those that exhibit the form 
of a ring. Of these, but few specimens are known. One 
of the most striking may be found about 6° below Mizar, 

455. W T hat are resolvable nebula)? How when seen through powerful 
telescopes ? 

456. Irresolvable nebula) ? Are any nebula) really irresolvable ? Remarks 
from Dr. Scoresby ? 

457. What further description of nebula) ? Specimen ? 

45S, What are annular nebulae? Are they common f What specimen in 
put 2 Pescribe it, 




NjSBUL.E. 



203 



the middle star in the tail of the Great Bear. It 
consists of a lar^e and 



ANNULAR NEBULAE. 




bright globular nebula, 
surrounded by a double 
ring, at a considerable 
distance from the globe; 
or rather a. single ring 
divided through about 
two -fifths of its circum- 
ference, and having one 
portion turned up, as it 
were, out of the plane of 
the rest. A faint nebu- 
lous atmosphere, and a 
small round nebula near 
it, like a satellite, com- 
pletes the figure. 

459. Another very conspicuous nebula of this class 
may be found half-way between /3 and 7, in the Lyre, 
and may be seen with a telescope of moderate power. 
It is small, and particularly well defined, so as, in fact, to 
have much more the appearance of a flat oval solid ring, 
than of a nebula. The space within the ring is filled 
with a faint hazy light, uniformly spread over it, like a 
fine gauze stretched over a hoop. 

460. " Planetary J\ T ehtdce" says Dr. Herschel, " are 
very extraordinary objects. They have, as their name 
imports, exactly the appearance of planets— round or 
slightly oval discs — in some instances quite sharply ter- 
minated, in others a little hazy at the borders, and of a 
light exactly equable, or only a very little mottled, which, 
in some of them, approaches in vividness to that of the 
actual planets. Whatever be their nature, they must be 
of enormous magnitude." 

461. Stellar JVebulce, or Nebulous Stars, are such as 
present the appearance of a thin cloud, with a bright 
star in or near the center. They are round or oval- 

459. W T hat other annular nebulae ? Describe. 

460. Planetary nebulae ? Describe. 

4*51. Stellar nebulae ? Remarks of Professor Mitchel? 



204 



ASTRONOMY. 



STELLAR NEBULA. 




shaped, and look like a star with a burr around it, or a 
candle shining through horn. "It was an object of this 
kind," says Prof. Mitchel, " which 
first suggested to Sir W. Herschel 
his great theory of the formation 
of suns out of a nebulous fluid. 
He thought it impossible to ac- 
count for the central location of 
stars, surrounded by nebulous 
matter, in any way except by sup- 
posing this to be a sort of atmos- 
phere attracted to, and sustained 
in its spherical form.by, the power of the central body. 
I have examined specimens of these objects, and always 
with increasing wonder. Their magnitude must be enor- 
mous, as the stars are certainly not nearer than other 
stars ; and yet the circular halo around them is of a 
diameter easily measured, and proves them to have a 
circumference perhaps greater than the entire orbit of 
Neptune." 

462. One of 
the most remark- 
able nebula in 
all the heavens 
may be found 
around the mid- 
dle star in the 
sword of Orion. 
It is easily seen 
with a common 
telescope. It is 
shaped like the 
head of some 
animal — a fish, for instance — with its mouth open. Near 
the inner surface of this mouth are four stars, ranged in 
the form of a trapezium. It requires a good telescope 
to see four stars ; but, with powerful instruments, six are 
visible, instead of four. 



GEEAT NEBULA IN OPwION. 




462, Bcseribp the nebula of Orion? 
stars m it? 



WliCW situated? Shape? What 



NEBULJE. 205 



463. The sun is considered by astronomers as belong- 
ing to this class of nebulous stars ; and the Zodiacal 
Light (322 and 325) has been regarded as of the nature 
of the gaseous matter with which the nebulous stars are 
surrounded. It is supposed that if we were as far from 
the sun as from the stellar nebulae, he would appear to us 
only as a small and nebulous star ! 

464. Until recently, the most powerful instruments 
have failed to reveal any thing like distinct stars, as com- 
posing the body of the remarkable nebula in Orion. 
Both the Herschels regarded it as positively irresolvable ; 
or, in other words, as composed of nebulous fluid or un- 
organized matter. But it has recently been seen to be 
composed of distinct stars, both by the monster telescope 
of Lord Eosse, and the great refractor of Cambridge, 
near Boston. 

465. The magnitude of this nebula must be beyond 
all human conception. " If," says Mr. Smyth, " the 
parallax of this nebula be no greater than that of the 
stars, its breadth cannot be less than a hundred times 
that of the diameter of the earth's orbit ; but if, as is 
more probable, it is a vast distance beyond them, its 
magnitude must be utterly inconceivable." 

466. Prof. Mitchel observes, that in case light be not 
absorbed in its journey through the celestial spaces, the 
light of the nebula of Orion cannot reach the eye in less 
than 60,000 years, with a velocity of twelve millions of 
miles in every minute of time! And yet this object 
may be seen from this stupendous distance, even by the 
naked eye ! What, then, must be its dimensions ? Here, 
indeed, we behold a universe of itself too vast for the 
imagination to grasp, and yet so remote as to appear a 
taint spot upon the sky." 

467. The number of such nebulous bodies is unknown, 



463. Kemarks respecting the sun ? 

464. How the nebula in Orion regarded? What recent discovery ? 

465. Its probable magnitude ? Remark of Smyth ? 

466. Prof. Mitchel's observations respecting its distance and dimensions ? 

467. What said of the number of nebulous bodies in the heavens ? Where 
most abundant \ JlerscheVs catalogue? Various forms? 



206 ASTRONOMY. 



perhaps we should say innumerable. They are especially 
abundant in the Galaxy or Milky Way. Sir W. Iler- 
schei arranged a catalogue, showing the places of two 
thousand of these objects. They are of all shapes and 
sizes, and of all degrees of brightness, from the faintest 
milky appearance to the light of a fixed star. 

468. Star Dust is a name given to those exceedingly 
faint nebulous patches that appear to be scattered about 
at random in the far-distant heavens. It is barely visible 
through the best telescopes, and seems to form a sort of 
back-ground, far beyond all stars, clusters, and nebulas, 
resolvable or irresolvable. 

469. " The nebulae," says Sir John Herschel, " fur- 
nish, in every point of view, an inexhaustible field of 
speculation and conjecture. That by far a larger share 
of them consist of stars, there can be little doubt ; and 
in the interminable range of system upon system, and 
firmament upon firmament, which we thus catch a 
glimpse of, the imagination is bewildered and lost." 

470. It is a general belief among astronomers that the 
material universe consists of distinct clusters, separated 
from each other by innumerable chasms : that the fixed 
stars by which we are surrounded constitute one great 
cluster — the sun being a star with the rest, and appearing 
as he does to us, solely on account of our nearness to him ; 
that the nebulae are far beyond our cluster, like so many 
distinct continents in the boundless ocean of immensity. 

471. Could we leave our system, and pass outward 
toward the fixed stars, they would doubtless expand to 
the dimensions of suns as we approached them, while 
our own central luminary w T ould dwindle to a glimmering 
star. Reaching the frontier of the cluster, and plunging 
oft' into the awful solitudes of space, toward the distant 
nebulae beyond, we should see them also expand as we 
drew near, while our vast firmament of stars seemed to 

468. What is meant by star dust f Wnere supposed to be situated ? 

469. Herschel's remark respecting the nebulae \ 

470. What the prevailing opinion among astronomers, as to the structure 
of the universe ? 

471. What imaginary journey and scenery described by the author ? 



NEBUUE. 207 



be gathering into a compact group ; till at length, enter- 
ing the bosom of the distant nebufe, we should find our- 
selves surrounded by new and strange constellations ; 
and if we saw our own firmament at all, should see it 
only as a faint annular nebula, far beyond the reach of 
all unassisted vision. 

472. The great stellar cluster in which the sun and 
solar system are imbedded is supposed, in its form, to 
resemble a double convex lens, with the sun and solar 
system near its center ; and by being viewed edgewise 
from our central position, to give us the phenomenon of 
the Milky Way. 

GREAT NEBULA OF THE SOLAR SYSTEM. 



The above is an edgewise view of the great stellar cluster, in the midst of which tho 
solar system is placed, as drawn by Sir William Herschel. Its figure was ascertained by 
gauging the space-penetrating power of his telescope, and then " sounding the heavens/' 
to ascertain the distance through the cluster, in all directions, to the open void. The 
nebulee lie in distinct and independent islands, far beyond the limits of our cluster. 

Let the student imagine the sun to be one of the stars near the middle of the lens- 
shaped cluster, of which the above is an edge view, with the planets revolving close 
around it. If, then, he look out upon the surrounding stars, the number visible, and 
their distinctness, will depend upon the direction in which he looks. If toward the 
thin part of the cluster (either up or down in the cut), fewer stars will be seen, while 
they will be comparatively distinct But if the view be toward the edge of the cluster, 
instead of the sides (or horizontally, in the cut), there will be seen beyond the large 
stars, and fading away to an indistinct and mingled light, a numberless host of stars ; 
and this zone of distant stars will extend quite around the heavens. Such is the Galaxy 
or Milky Way. The zone of milky light is the light of the stars in the remote edge of 
the great cluster. The opening in theleft end of the figure is a split in the cluster, and 
constitutes the division seen in the milky way, extending part way around the heavens. 
See cut paire 203. 

The vastf apparent extent of the Galaxy, as compared with other nebula?, is supposed 
to be justly attributable to its comparative nearness. Were we as far from the solar 
system as from the nebulas in the Lyre, the Milky Way would doubtless appear as an 
annular nebula no larger than that. It may therefore with propriety be called "the 
great nebula of the solar system." 

473. Sir W. Herschel estimated that 50,000 stars 
passed the field of his telescope, in the Milky Waj^, in a 

472. Supposed form of our own stellar cluster ? Philosophy of Galaxy ? 
(Why apparently so lanje ? How appear at a great distance ?) 

473. Stars in Milky Way ? Mutual distances ? Character of each star ? 



208 ASTRONOMY. 



single hour ! And yet the space thus examined was 
hardly a point in the mighty concave of our own " sun- 
strown firmament." "What an idea is here conveyed to 
the mind, of the almost boundless extent of the uni- 
verse ! The mutual distances of these innumerable orbs 
are probably not less than the distance from our sun to 
the nearest fixed stars, while they are each the center of 
a distinct system of worlds, to which they dispense light 
and heat. 

474. Were the universe limited to the Great Solar 
Cluster, in the midst of which we are placed, it would 
be impossible to conceive of its almost infinite dimen- 
sions ; but when we reflect that this vast and glowing 
zone of suns is but one of thousands of such assem- 
blages, which, from their remoteness, appear only as 
fleecy clouds hovering over the frontiers of space, we are 
absolutely overwhelmed and lost in the mighty abyss of 
being ! 

475/ And here we close our rapid and necessarily im- 
perfect survey of the Sidereal Heavens. And while the 
mind of the student is filled with awe, in contemplating 
the vastness and majesty of creation, let him not forget 
that over all these Jehovah reigns — that " these are but 
parts of his. ways ;" and yet so perfect is his knowledge 
and providence in every world, that ' the very hairs of 
our heads are numbered, and not a sparrow falls without 
his notice. And while we behold the wisdom, power, 
and goodness of God so gloriously inscribed in the heav- 
ens, let us learn to be humble and obedient — to love and 
serve our Maker here — that we may be prepared for the 
still more extended scenes of another life, and for the 
society of the wise and good in a world to come. 

474. Magnitude of our own cluster ? What in comparison with all others? 

475. Remarks in closing paragraph ? Moral reflections ? 



PART III. 

PRACTICAL ASTRONOMY. 



■+++- 



CHAPTER I 



PROPERTIES OF LIGHT 



4:76. Practical Astronomy has respect to the means 
employed for the acquisition of astronomical knowledge. 
It includes the properties of light, the structure and use 
of instruments, and the processes of mathematical calcu- 
lation. 

In the present treatise, nothing further will be attempted than a mere introduction to 
practical astronomy. In a work designed for popular use, mathematical demonstrations 
would be out of place. Still, every student in astronomy should know how telescopes 
are made, upon what laws they depend for their power, and how they are used. It is 
for this purpose mainly that we add the following chapters on Practical Astronomy. 

477. Light is that invisible ethereal substance by whi'ch 
we are apprised of the existence, forms, and colors of 
material objects, through the medium of the visual organs. 
To this subtile fluid we are especially indebted for our 
knowledge of those distant worlds that are the principal 
subjects of astronomical inquiry. 

478. The term light is used in two different senses. It 
may signify either light itself, or the degree of light by 
which we are enabled to see objects distinctly. In this 
last sense, we put light in opposition to darkness. But 

476. Parts of the book gone over ? Subject of Part III. ? Of Chapter I. ? 
What is practical astronomy ? (How far discussed in this treatise ?) 

477. Define light. For what indebted to it ? 

478. Different senses in which the term is used ? What is darkness ? Can 
it be dark and light at the same time? Is there any place without light ? 
^Quotation from Dick ?) 



210 ASTRONOMY. 



it should be borne in mind that darkness is merely the 
absence of that degree of light which is necessary to 
human vision ; and when it is dark to us, it may be light 
to many of the lower animals. Indeed, there is more or 
less light even in the darkest night, and in the deepest 
dungeon. 

" Those unfortunate individuals," says Dr. Dick, " who have been confined in the dark- 
est dungeons, have declared, that though, on their first entrance, no object could be per- 
ceived, perhaps for a day or two, yet, in the course of time, as the pupils of their eyes 
expanded, they could readily perceive mice, rats, and other animals that infested their 
cells, and likewise the walls of their apartments ; which shows that, even in such situa- 
tions, light is present, and produces a certain degree of influence.'" 

479. Of the nature of the substance we call light two 
theories have been advanced. The first is, that the whole 
sphere of the universe is filled with a subtile fluid, w T hich 
receives from luminous bodies an agitation ; so that, by 
its continued vibratory motion, we are enabled to per- 
ceive luminous bodies. This w T as the opinion of Des- 
cartes, Euler, Huygens, and Franklin. 

The second theoiy is, that light consists of particles 
thrown off from luminous bodies, and actually proceeding 
through space. This is the doctrine of Newton, and of 
the British philosophers generally. 

Without attempting to decide, in this place, upon the relative merits of these two hy- 
potheses, we shall use those terms, for convenience sake, that indicate the actual passage 
of light from one body to another. 

480. Light proceeds from luminous bodies in straight 
lines, and in all directions. It will not wind its way 
through a crooked passage, like sound ; neither is it con- 
fined to a part of th.Q circumference around it. 

As the sun may be seen from every point in the solar system, and far hence into spaco 
in every direction, even till he appears but a faint and glimmering star, it is evident that 
he fills every part of this vast space with his beams. And the same might be said of 
every star in the firmament. 

481. As vision depends not upon the existence of light 
nierely, but requires a certain degree of light to emanate 
from the object, and to enter the pupil of the eye, it is 
obvious that if we can, by any means, concentrate the 



479. What theories of the nature of light, and by whom supported respect- 
ively ? (Remark of author ?) 

480. How light proceeds from luminous bodies ? (Radiations from sun and 
stars ?) 

481. How improve vision, and why? (Animals ?) 



REFRACTION OF LIGHT. 



211 



light, so that more may enter the eye, it will improve 
our perception of visible objects, and even enable us to 
see objects otherwise wholly invisible. 

Some animals have the power of adapting their eyes to the existing degree of light. 
The cat, horse, &c., can see day or night; while the owl, that sees well in the night, sees 
poorly in the day-time. 

482, Light may be turned out of its course either by 
reflection or refraction. It is reflected when it falls upon 
the highly polished surface of metals and other intrans- 
parent substances ; and refracted when it passes through 
transparent substances of different densities. 



REFRACTION OF LIGHT. 

483. Whenever light passes from a rare medium to 
one more dense, and enters the latter obliquely, it inva- 
riably leaves its first direction, and assumes a new one. 
This change or bending of the rays of light is what is 
called Refraction. 

The term refract is from the Latin re, and frango, to break ; and signifies the break- 
ing of the natural course of the rays. 

484. As air and water are both transparent, but of 
different densities, it follows that, when light passes 
obliquely from one to LIGHT KEFRACTE1) BY WATKR . 

the other, it will be 
refracted. If it pass 
from the air into the 
water, it will be re- 
fracted toward a per- 
jpendiciilar. 

Here the ray xi C strikes the 
water perpendicularly, and passes 
directly through to B without 
being refracted. But the ray D C 
strikes the water at C obliquely ; 
and instead of passing straight 
through to E, is refracted at C, 
and reaches the bottom of the 

water at F. If, therefore, a person were to receive the ray into the eye at F, and to 
iudge of the place of the object from which the light emanates from the direction of the 
ray"C F, he would conclude that he saw the object at G, unless he made allowance for 
the refraction of the light at C. 




482. How light turned out of course ? 

4S3. What is refraction? How produced ? (Derivation of term refract?) 

484. How refracted by air and water? (Illustrate by diagram.) 



212 



ASTRONOMY. 



LIGHT PROCEEDING FROM WATER. 



485. When light 
passes obliquely 
from a denser to a 
rarer medium, as 
from water into air, 
it is refracted from 
a perpendicular to- 
ward a horizontal. 

Here the lamp A shines up 
through water into air. The 
ray that strikes the surface per- 
pendicularly passes on to B 
without being refracted; but 
the other rays that leave the water obliquely are refracted toward a horizontal direction, 
in proportion to their distance from the perpendicular; or, in other words, in propor- 
tion to the obliquity of their contact with the surface of the water. 

486. In consequence of the refraction of light toward 
a horizontal direction, in passing from water into air, a 
pole, half of which is in the water, seems bent at the 
surface, and the lower end seems nearer the surface than 
it really is. For the 




EFFECT OF REFRACTION. 



same reason, the bottom 
of a river seems higher, 
if seen obliquely, than it 
really is ; and the water 
is always deeper than 
we judge it to be. 

In this cut, the oar, the blade of 
which is in the water, seems bent at 
the surface of the water. The rays of 
light passing from the part under 
water to the^surface at D, are refract- 
ed toward a horizontal direction at 

that point, and received into the eye of the observer at B, who, judging of the position 
of the immersed portion of the oar from the direction of the rays D B, locates the blade 
of the oar at C; thus reversing the effect illustrated at 4S4. 

487. The refracting power of different transparent 
substances depends mainly upon their density. Water 
refracts more than air, glass more than water, and dia- 
mond most of all. But the angle of incidence, or the 
obliquity of the contact of the rays with the denser sub- 




485. How when light passes from denser to rarer mediums ? (Diagram.) 

486. Effect of refraction upon objects seen under water? (Diagram.) 

487. Upon what does the refracting power of different transparent media 
epend ? 



KEFKACTION OF LIGHT. 



213 



EFFECT OF REFRACTION. 

c 





stance, has also much to do in determining the amount 
of refraction. 

488. By the aid of re- 
fraction, we may see ob- 
jects that are actually be- 
hind an opake or intrans- 
parent body. 

Here the piece of money at A, at the 
bottom of the cup, would be invisible to 
the beholder at B, if the cup was empty, 
as the light from the money would pass 
from A to C ; but when the cup is filled 
with water, the light is refracted to B, 
and the beholder sees the money appa- 
rently at D. 

489. By the law of refraction, light has been found 
to consist of a combination of colors. By passing a 
beam of light through a triangular piece of flint glass 
called a prism, it is seen that some parts of the light are 
more refrangible than others, so that the light is analyzed, 
or separated into its component parts or elements. 

BEFRACTION BY A PRISM. 



Violet 

Indigo 

Blue 

Green. 

Yellow 

Orange 

Bed/:. 




White & 



Let a ray of light from the sun be admitted through a hole in the window shutter, A, 
into a room from which all other light is excluded ; it will form, on a screen placed a lit- 
tle distance in front, a circular image, B, of white light. Now, interpose near the shut- 
ter a glass prism, C, and the light, in passing through it, will not only be refracted in the 
same "direction, both when it enters the prism and when it leaves it, but the several rays 
of which white light is composed will be separated, and will arrange in regular order on 
the screen, immediately above the image B, which will disappear. The violet ray, it 
will be seen, is most refracted, and the red least ; the whole forming on the screen an 
elongated image of the sun, called the solar spectrum. — Johnston. 

48S. What other effect of refraction ? (How illustrated ?) 
4S9. What discovery by refraction ? (How made ?) 



214 



ASTRONOMY 



490. It is the refraction of the clouds that gives the 
sky its beautiful colors morning and evening ; and the 
refracting power of the rain-drops produces the beautiful 
phenomenon of the rainbow. 



ATMOSPHERICAL REFRACTION. 

491. The refracting power of the atmosphere produces 
many curious phenomena. Sometimes ships are seen 
bottom upwards in the air, single or double. At other 
times, objects really below the horizon, as ships or 
islands, seem to rise up, and to come distinctly in view. 

492. A very important effect of refraction, as it relates 
to astronomy, is, that it more or less affects the apparent 
places of all the heavenly bodies. As the light coming 
from them strikes the atmosphere obliquely, and passes 
downward through it, it is refracted or bent toward the 
earth, or toward a perpendicular. And as we judge of 
the position of the object by the direction of the ray 
when it enters the eye, we place objects higher in the 
heavens than they really are. 

ATMOSPHERICAL REFRACTION. 




Let A, in the cut, represent the earth ; B, the atmosphere ; C C, the visible horizon ; 
and the exterior circle the apparent concave of the heavens. Now, as the light passes 
from the stars, and strikes the atmosphere, it is seen to curve downward, because it 
strikes the atmosphere obliquely; and the air increases in density as we approach the 
earth. But as the amount of refraction depends not only upon the density, but also 
upon the obliquity of the contact, it is seen that the refraction is greatest at the horizon, 
and gradually diminishes till the object reaches the zenith, when there is no obliquity, and 
the refraction wholly ceases. The dark lines in the cut show the true, and the dotted 
the apparent positions. 



490. What other effects of refraction ? 

491. Atmospherical refraction? Effect's on terrestrial objects? 

492. Upon apparent places of stars, &c. ? (Diagram. What said of exag- 
geration f) 



ATMOSPHERICAL REFRACTION. 215 



In the cut, the depth of the atmosphere, as compared with the globe, is greatly exag- 
gerated. Even allowing it to be 50 miles deep, it is only ^th of the semi-diameter of 
the globe, which is equal to only about y-^th of an inch upon a common 13-inch globe. 
But it was necessary to exaggerate, in order to illustrate the principle. 

493. The amount of displacement of objects in the 
horizon, by atmospherical refraction, is about 33', or a 
little more than the greatest apparent diameter of either 
the sun or moon. It follows, therefore, that when we 
see the lower edge of either apparently resting on the 
horizon, its whole disk is in reality below it ; and would be 
entirely concealed by the convexity of the earth, were it 
not for refraction. 

494. Refraction sometimes causes the sun and moon 
to appear elongated horizontally, when near the horizon, 
and seen through a dense atmosphere. The rays from 
their lower limb being refracted more than those from 
the upper limb, on account of coming to us through a 
lower and denser portion of the atmosphere, the lower 
portion seems higher in proportion ; or, in other words, 
the perpendicular diameter of the object seems the 
shortest. It is then called a horizontal moon. 

495. Another effect of refraction is, that the sun seems 
to arise about three minutes earlier, and to set about three 
minutes later, on account of atmospherical refraction, 
than it otherwise would ; thus adding about six minutes, 
on an average, to the length of each clay. 

The atmosphere is said to be so dense about the North Pole as to bring the sun above 
the horizon some days before he should appear, according to calculation. In 1596, some 
Dutch navigators, who wintered at Nova Zembla, in latitude 76°, found that the sun be- 
gan to be visible 17 days before it should have appeared by calculation ; and Kepler 
computes that the atmospheric refraction must have amounted to 5°, or 10 times as 
much as with us. 

496. The twilight of morning and evening is produced 
partly by refraction, but mainly by reflection. In the 
morning, when the sun arrives within 18° of the horizon, 
liis rays pass over our heads into the higher region of 
the atmosphere, and are thence reflected down to the 
earth. ,The day is then said to dawn, and the light 
gradually increases till sunrise. In the evening, this 

493. Amount of displacement of celestial objects by refraction ? W r hat 
follows ? 

494. What effect upon apparent form of moon, &c. ? 

495. On length of days ? (How about North Pole ?) 

496. Cause of twiliglit? (Note.) 



216 



ASTRONOMY. 



process is reversed, and the twilight lingers till the sun 
is 18° below the horizon. There is thus more than an 
hour of twilight both morning and evening. 

In the arctic regions, the sun is never more than 18° below the horizon; so that the 
twilight continues during the whole night. 

497. In making astronomical observations, for the pur- 
poses of navigation, &c 5 allowance has to be made for 
refraction, according to the altitude of the object, and 
the state of the atmosphere. For this purpose tables 
are constructed, showing the amount of refraction for 
every degree of altitude, from the horizon to the zenith. 




REFRACTION BY GLASS LENSES. 

498. A lens is a piece of glass or other transparent 
substance, of such a form as to collect or disperse the 
rays of light that are passed through it, by refracting 
them out of a direct course. They are of different forms, 
and have different powers. lenses op difpeeent F0Bm 

In the adjoining cut, we have an edgewise 
view of six different lenses. 

A is a plano-convex, or half a double con- 
vex lens ; one side being convex, and the other 
plane. 

B is a plano-concave ; one surface being con- 
cave, and the other plane. 

Q is a double-convex lens, or one that is 
bounded by two convex surfaces. 

D is a double-concave lens, or a circular 
piece of glass hollowed out on both sides. 

E is a concavo-convex lens, whose curves differ, but do not meet, if produced. 

F is a miniscus, or a concavo-convex lens, the curves of whose surfaces meet. 

499. A double-convex 
lens converges parallel 
rays to a point called 
the focus ; and the dis- 
tance of the focus de- 
pends upon the degree 
of convexity. 

In the first of these cuts, the lens 
is quite thick, and the focus of the 
rays is quite near; but the other 
being less convex, the focus is more 
remote. 



LIGHT REFBACTED BY LENSES. 




497. What allowance for refraction ? Tables ? 

498. What is a lens f (Draw and describe different, kinds ?) 

499. Refracting power of double-convex lens? Focal distance ? (Diagram, 
and illustrate.) 



ATMOSPHERICAL REFRACTION. 



217 




500. The distance of 
the focus of a double-con- 
vex glass lens is the ra- 
dius of the sphere of its 
convexity. 

In this cut, it will be seen that the 
parallel rays A are refracted to a focus 
at C, by the double convex lens B, the 
convexity of whose surfaces is just 
equal to the curve of the circle D. 

501. The focal distance of a plano-convex lens is equal' 
to the diameter of the sphere formed by the convex surface 

i ' PLANO-CONCAVE—FOCAL DISTANCE. 

It must be borne in mind 
that light is refracted both 
when it enters and when it 
leaves a double-convex lens, 
and in both instances in the 
same direction ; and, so far as 
the distance of the focus is con- 
cerned, to the same extent. 
But when the lens is convex 
only on one side, half its re- 
fracting power is gone, so that 
the rays are not so soon re- 
fracted to a focus. In this case, 
the focal distance is equal to 
the diameter of the sphere 
formed by extending the convex surface of the lens ; while with the double-convex lens, 
the focal distance is only equal to the radius of such sphere. In the cut, the parallel 
rays A are refracted to a focus at B, by the plano-concave lens C ; and the distance C B 
is the diameter of the circle D, formed by the convex surface of the lens C produced. 

502. A double- 
concave lens dis- 
perses parallel 
rays, as if they 
diverged from the 
center of a circle 
formed by the con- 
vex surface pro- 
duced. 

In this cut, the parallel rays A are dispersed by the double-concave lens B, as shown 
at C; and their direction, as thus refracted, is the same as if they proceeded from the 
point D, which is the center of a circle formed by the concave surface of the lens pro- 
duced. 




KAYS DISPERSED BY EEFEACTION. 




500. How focal distance governed ? (Diagram.) 

501. What is the focal distance of a plano-convex lens ? (Diagram.) 

502. Effect of double-convex lens ? Amount of divergency of rays ? (Dia- 
gram.) 

10 



218 



ASTRONOMY. 



BUBNING-GLASS. 



503. Common spectacles, opera-glasses, burning-glasses, 
and refracting telescopes are made by converging light 
to a focus, by the use of double-convex lenses. 

The ordinary burning-glass, which may be bought for a 
few shillings, is a double-convex disk of glass two or three 
inches in diameter, inclosed in a slight metallic frame, 
with a handle on one side. 
Old tobacco-smokers some- 
times carry them in their 
pockets, to light their pipes 
with when the sun shines. 
In other instances, they have 
been so placed as to fire a 
cannon in clear weather, by 
igniting the priming at 12 
o'clock. 



The adjoining cut represents a large burn- 
ing-glass converging the rays of the sun to a 
focus, and setting combustible substances on 
fire. Such glasses have been made power- 
ful enough to melt the most refractory sub- 
stances, as platinum, agate, &c. " A lens three feet in diameter," says Professor Gray, 
" has been known to melt carnelian in 75 seconds, and a piece of white agate in 30 
seconds." 




REFLECTION OF LIGHT. 

504. We have now shown how light may be turned 
out of its course, and analyzed, dispersed, or converged 
to a point by refraction. Let us now consider how it 
may be converged to a focus by reflection. 

505. When light falls upon a highly polished surface, 
especially of metals, it is reflected or thrown off in a new 
direction, and the angles of con- 
tact and departure are always 
equal. 

Let AB represent the polished metallic sur- 
face. C the source of light, and the arrows the 
direction of the ray. Then D would represent 
the angle of incidence or contact, and E the angle 
of reflection or departure — which angles are seen 
to be equal. 



REFLECTION BY A PLANE MIREOE. 



A— & 




503. What articles made with double-convex lenses ? Uses ? (Power of 
burning-glasses ?) 

504. What now shown in this chapter ? What next ? 

505. What is reflection, and when does it take place ? What law governs 
it ? (Diagram.) 



REFLECTION OF LIGHT. 



219 



506. A concave mirror reflects parallel rays back to a 
focus, the distance of which is equal to half the radius 
of the sphere formed by the concave surface produced. 



REFLECTION BY A CONCAVE MIRROR. 



B^/ 








A 




M^ 


M\ \ \ 


ff^W \ 


www \ 


S\ w\\ \ 


||^>0^\ 




r* 












I — — =df C — ; i 


m^^^^^w 










1 


m^^s/// 


m^//// 










/ 


^/ /// ' 


W/// _j 


W/ / 










/ 


Iv 










/ 



B 




In this cut, the parallel rays A fall upon the concave mirror B B, and are reflected to 
the focus C, which is half the radius of the sphere formed by the surface of the mirror 
produced. If, therefore, it was desirable to construct a concave mirror, having ^its focus 
10 feet distant, it would only be necessary to grind it on the circle of a sphere having a 
radius of 20 feet. 

507. In reflection, a portion of the light is absorbed or 
otherwise lost, so that a reflector of a given diameter will 
not converge as much light to a focus as a double-convex 
lens of the same size. In the latter case, all the light is 
transmitted. Still, reflectors have been formed of such 
power as to melt iron, and other more difficult sub- 
stances. 

We have now considered so much of optics as is necessary to an nn derst andi n^ ^ ^ 
principles upon which telescopes are constructed ; and, for further particulars, shall refer 
the student to books of Natural Philosophy. 

506. How does a concave mirror reflect parallel rays? Distance of focus ? 
(Diagram. How would you construct a concave mirror with a 10 feet focus .; 

507. Is all the light falling upon a polished surface reflected ? W hat then f 
(Closing note ?) 



220 ASTRONOMY. 



CHAPTER II 



TELESCOPES. 



508. A Telescope is an optical instrument employed in 
viewing distant objects, especially the heavenly bodies. 
The term telescope is derived from two Greek words, viz., 
tele, at a distance, and sJcqpeo, to see. 

509. So far as is now known, the ancients had no 
knowledge of the telescope. Its invention, which oc- 
curred in 1609, is usually attributed to Galileo, a phi- 
losopher of Florence, in Italy. 

The discovery of the principle upon which the refracting telescope is constructed 
was purely accidental. The children of one Jansen, a spectacle-maker of Middlebnrgb, 
in Holland, being at play in their father's shop, happened to place two glasses in such a 
manner, that in looking through them, at the weather-cock of the church, it appeared 
to be nearer and much larger than usual. This led their father to fix the glasses upon a 
board, that they might be ready for observation ; and the news of the discovery was soon 
conveyed to the learned throughout Europe. Galileo hearing of the phenomenon, soon 
discovered the secret, and put the glasses in a tube, instead of on a board ; and thus the 
first telescope was constructed. 

510. The telescope of Galileo was but one inch in di- 
ameter, and magnified objects but 30 times. Yet with 
this simple instrument he discovered the face of the 
moon to be full of inequalities, like mountains and val- 
leys ; the spots on the sun ; the phases of Venus ; the satel- 
lites of Jupiter ; and thousands of new stars in all parts 
of the heavens. 

Notwithstanding this propitious commencement, so slow was the progress of the 
telescope toward its present state, that in 1816, Bonnycastle .speaks of the 80-fold mag- 
nifying power of the telescope of Galileo as M nearly the greatest perfection that this 
kind of telescope is capable of!" 

511. If he be the real author of an invention who, 
from a knowledge of the cause upon which it depends, 
deduces it from one principle to another, till he arrives 

508. Subject of Chap. II. ? Telescope ? Derivation ? 

509. Ancient or modern ? Inventor ? (Incidents of discovery ?) 

510. Galileo's telescope? Discoveries with it? (Progress in telescope 
making ?) 

511. Is Galileo entitled to the honor of inventing the telescope? (Where 
is his ?) 



DIFFERENT KINDS OF TELESCOPES. 221 



at the end proposed, then the whole merit of the inven- 
tion of the telescope belongs to Galileo. The telescope 
of Jansen was a rude instrument of mere curiosity, acci- 
dentally arranged ; but Galileo was the first who con- 
structed it upon principles of science, and showed the 
practical uses to which it might be applied. 

It is said that the original, telescope constructed by Galileo is still preserved in the 
British Museum. A piginy, indeed, in its way, but the honored progenitor of a race of 
giants ! 

512. The discovery of the telescope tended greatly to 
sustain the Copernican theory, which had just been pro- 
mulgated (10), and of which Galileo was an ardent dis- 
ciple. Like Copernicus, however, his doctrines subjected 
him to severe persecutions, and he was obliged to re- 
nounce them. 

The following is his renunciation, made June 28, 1633 : " I, Galileo, in the seventieth 
year of my age, on bended knees before your eminences, having before my eyes and 
touching with my hands the Holy Gospels, I curse and detest the error of the earth's 
movement 11 As'he left the court, however, after this forced renunciation, he is said 
to have stamped upon the earth, and exclaimed, " It does move, after all !" Ten years 
after this he was sent to prison for the same supposed error; and soon, his age advan- 
cing, the grave received him from the malice of his persecutors. 

DIFFERENT KINDS OF TELESCOPES. 

513. Telescopes are of two kinds — Reflectors and Re- 
fractors. Refracting telescopes are made by refracting 
the light to a focus with a glass lens (499) ; and reflect- 
ing telescopes, by reflecting it to a focus w T ith a concave 
mirror (506). Besides this general division, there are 
various kinds, both of reflectors and refractors. 

514. Telescopes assist vision in various ways — first, 
by enlarging the visual angle under which a distant ob- 
ject is seen, and thus magnifying that object ; and, 
secondly, by converging to a point more light than could 
otherwise enter the eye — thus rendering objects distinct 
or visible that would otherwise be indistinct or invisible. 

All the light falling upon a six or a twelve inch lens may be converged to a focus, so 
as to be taken into the human eye through the pupil, which is but one~-fourth of an inch 
in diameter. Our vision is thus made as perfect by art as if nature had given us ability 
to enlarge the eye till the pupil was a foot in diameter. 

512. Kelation of discovery to Copernican theory ? Effects upon Galileo i 
(His renunciation ? Death ?) 

513. Kinds of telescopes ? Describe. 

514. How assist vision ? (Illustrative uote ?) 



222 



ASTRONOMY. 



515. Refracting telescopes may consist of a double- 
convex lens placed upon a stand, without tube or eye- 
piece. Indeed, a pair of ordinary spectacles is nothing 
less than a pair of small telescopes, for aiding impaired 



vision. 



REFRACTING TELESCOPE WITH A SINGLE LENS. 




Here the parallel rays are seen to pass through the lens at A, and to be so converged to 
a point as to enter the eye of the beholder at B. His eye is thus virtually enlarged to the 
size of the lens at A. But it would be very difficult to direct such a telescope toward 
celestial objects, or to get the eye in the focus after it was thus directed. 

516. The Galilea/h telescope consists of two glasses — 
a double-convex next the object, and a double-concave 
near the eye. The former converges the light till it can 
be received by a small double-concave, by which the con- 
vergency is corrected (502), and the rays rendered paral- 
lel again, though in so small a beam as to be capable of 
entering the eye. 

GALILEAN TELESCOPE. 




Here the light is converged by the lens A, till it can be received by the double-con- 
cave lens B, by which the rays are made to become a small parallel beam, that can enter 
the eye at C. This was the form of the telescope constructed by Jansen, and improved 
by Galileo ; on which account it is called the Galilean telescope. In the cut, the two 
lenses are represented as fastened to a board, as first exhibited by Jansen. 

517. The common astronomical telescope consists of 
two glasses — viz., a large double-convex lens next the 

515. Simplest form of refracting telescope ? (Diagram ?) 

516. Galilean telescope ? (Diagram and explanation? Why called Gall- 
lean ?) 

517. How common astronomical telescopes made ? Why in tube ? 



DIFFERENT KINDS OF TELESCOPES. 



223 



object, called the object-glass / and a small double-convex 
lens or microscope next the eye, called the eye-piece. For 
the greater convenience in using, they are both placed in 
a tube of wood or metal, and mounted in various ways, 
according to their size, and the purposes to which they 
are devoted. 



LENSES PLACED IN A TUBE. 




EBFRACTING TELESCOPE MOUNTED ON A STAND. 



A is the object-glass, B the eye-piece, and C the place where the tube in which the 
eye-piece is set. slides in and out of the large tube, to adjust the eye-piece to the focal 
distance. By placing the lenses in a tube, the eye is easily placed in the focus, and the 
object-glass directed toward any desired object. 

518. The object-glass of a telescope is usually pro- 
tected, when not in use, by a brass cap that shuts over 
the end of the instrument ; and the eye-pieces, of which 
there are several, of different magnifying powers, are 
so fixed as to screw 
into a small movable 
tube in the lower end 
of the instrument, so 
as to adjust them re- 
spectively to the fo- 
cus, and to the eyes 
of different observ- 
ers. Such telescopes 
usually represent ob- 
jects in an inverted 
position. 

The adjoining cut represents 
the simplest form of a mounted 
refractor. The object-glass is at 
A, where the brass cap may be 
seen covering it. B is the small 
tube into which the eye-piece 
is screwed, and which is moved 
in and out by the small screw 
C. Two eye-pieces may be seen 
at D — one short one, for astro- 
nomical observations; and a 
long one, for land objects. For 

viewing the sun, it is necessary to add a screen, made of colored glass. At E, a bolt goes 
into a socket in the top of the stand, in which it turns, allowing the telescope to sweep 




518. How object-glass protected? What said of eye-pieces? (Cut and 
explanation ?) 



224 ASTRONOMY. 



around the horizon ; while the joint, connecting the saddle in which the telescope rests 
with the top of the bolt, allows it to be directed to any point between the horizon and 
the zenith. But such stands answer only for comparatively small instruments. 

519. Refracting telescopes are mounted in various 
ways. So important is it that they should not shake or 
vibrate, that, in most observatories, the stand rests upon 
heavy mason-work in no way connected with the build- 
ing, so that neither the wind nor the tread of the ob- 
server can shake it. They are then furnished with a 
double axis, which allows of motion up and down, or 
east and west ; and two graduated circles show the pre- 
cise amount of declination and right ascension. They 
are then furnished with clockwork, by which the tele- 
scope is made to move westward just as fast as the earth 
turns eastward ; so that the celestial object being once 
found, by setting the instrument for its right ascension 
and declination, or by the aid of the Finder— -a small 
telescope attached to the lower end of the large one — it 
may be kept in view by the clockwork for any desirable 
length of time. A telescope thus furnished with right 
ascension and declination circles is called an Equatorial, 
or is said to be equatorially mounted, because it sweeps 
east and west in the heavens parallel to the equator. 

520. The object-glasses of telescopes are not always 
made of a single piece of glass. They may be made of 
two concavo-convex glasses, like two watch crystals, with 
their concave sides toward each other, or with a thin 
double concave glass between them. They are thus 
double, or triple ; but when thus constructed, the whole 
is called a lens, as if composed of a single piece. Lenses 
have also been formed by putting two concavo-convex 
glasses together, and filling the space between them with 
some transparent fluid. These are called Barlow lenses, 
from Prof Barlow, their inventor. 

521. As a prism analyzes the light, and exhibits dif- 
ferent colors, so a double-convex lens may analyze the 



519. How refractors mounted, and why ? When equatorial, and why ? 

520. How object-glasses made ? What a lens ? A Barlow lens ? 

521. What is an Achromatic telescope ? (Derivation of term?) 



DIFFERENT KINDS OF TELESCOPES. 



225 



light that falls near its circumference, and thus represent 
the outside of the heavenly bodies as colored. But this 
defect is remedied by using disks made of different kinds 
of glass, so as to correct one refraction by another. Re- 
fracting telescopes thus corrected are called Achromatic 
telescopes. 

Achromatic is from the Greek a chroma, which signifies destitute of color. Most 
refracting telescopes are now so constructed as to be achromatic. 

522. It is 'but recently that any good refracting tele- 
scopes have been made in this country. The best have 
formerly been made in Germany and France ; but they 
are now manufactured with success, and to considerable 
extent, by Mr. Henry Fitz, Jun., New York city. 

It is now (1S53) about seven years since Mr. Fitz commenced the manufacture of 
refracting telescopes, and thus far he has been very successful. At each fair of the 
American Institute, for seven years, he has received the highest premium — a gold 
medal. The glass used by him is obtained from Paris, because none suitable for large 
telescopes has yet been made in America. His telescopes are perfectly achromatic, and 
are sold much cheaper than imported ones of the same size and value. 

Mr. Fitz has recently made a very valuable improvement in the mounting of tele- 
scopes — one which is not only much superior to the old method, but which costs only 
about one-half as much. This improvement consists in using a single piece of cast-iron 
in the place of several pieces of brass work. It is very simple, secures great steadiness 
to the instrument, and is easily adjusted. 

The writer is fully satisfied of the value of this improvement, and would recommend 
it, as well as Mr. Fitz's instruments, to all institutions and amateur astronomers about 
to purchase either. Besides patronizing a worthy American optician, they will get as 
good a telescope and much better mounting than by sending abroad, and at far less ex- 
pense. The following is a list of telescopes manufactured by Mr. Fitz, with the prices 
attached. 



PPwICES OF FTTZ'S TELESCOPES, EQTTATOEIALLY MOUNTED, ETC. 



Focal length. 


Object-glass. 


New style 
mounting - . 


Old style 
mounting. 


Difference of cost. 


9 ft. 


Si inches. 


$1,400 


$2,300 


$900 


11 4i 


84: " 




2,220 




10 ft. 8 inch. 


8 


1.500 


2,000 


850 


8 " 


6| " 


TOO 


1,200 


500 


7 " 


5 " 


400 


750 


350 


7 " 


4£ " 


300 


500 


200 


5 " 


4 


225 


400 


175 



He will furnish a very good telescope of three inches aperture for $120, equatorially 
mounted, with eye-pieces, &c. The size priced at $225 is equal to that at Yale College. 
A good revolving dome for an observatory building can be built for $100. 

This note is inserted exclusively for the benefit of institutions using the work, and 
without any request or remuneration from Mr. Fitz. Orders or letters of inquiry may 
be addressed to Henry Fitz, Jun., 237 Third-street, New York. 



522. Where telescopes formerly made ? Where and by whom now, in thia 
country ? 

10* 



226 



ASTRONOMY. 




etjtherfobd's equatorial refractor. 

523. The above cut represents an equatorial telescope 
manufactured by Mr. Henry Fitz, of New York — the one 
used by the author in making most of his observations. 
Its object-glass is six inches in diameter, and its focal 
length eight feet. 

A is the declination circle, and B the circle of right ascension. The two sticks hang- 
ing from these circles are used to move the instrument in right ascension or declination, 
while the observer is at the eye-piece. 

The Finder is seen attached to the lower end of the large instrument. It takes in 
a larger field of view in the heavens than the latter, and enables the observer to look 
up objects with facility, and bring them into the field of the larger instrument 

This instrument has no clockwork attached. It rests upon a pillar of heavy mason- 
work, the top of which may be seen in the cut ; and in the hands of its present owner, 
Lewis M. Rutherford, Esq., has already rendered very efficient service. 

523. Yr. Rutherford's telescope? By whom made ? 



DIFFERENT KINDS OF TELESCOPES. 



227 




GREAT REFRACTING TELESCOPE AT CINCINNATI, OHIO. 

524. The above cut represents one of the most import- 
ant telescopes in the United States. It is located in the 
observatory on Mount Adams, near Cincinnati, Ohio, 
and has been for several years under the direction of 
Prof. 0. M. Mitchel, by whose instrumentality it was 
purchased and mounted. 

The object-glass is about 12 inches in diameter, with a focal distance of 17 feet. It 
was purchased in Munich, Germany, in 1S44, at an expense of nearly ten thousand dol- 
lars. There is but one larger than this in the United States, and but two larger in the 
world. 



524. Cincinnati refractor — where located ? By whom purchased ? (Where ? 
When ? Cost ? Size and focal distance ? Comparati ve size ?) 



228 



ASTRONOMY. 




THE GREAT C2AIG TELESCOPE, WANDSWOBTH COMMON, NEAE LONDON. 

525. This is the largest refracting telescope ever con- 
structed. The object-glass is two feet in diameter, with 
a focal distance of 76 feet. The tube is of heavy sheet 
iron, and shaped somewhat like a cigar. It is 13 feet in 
circumference in the largest place, and weighs about 
three tons. 

This telescope is suspended from a brick tower 65 feet high, 15 feet in diameter, and 
weighing 220 tons. The top of the tower, from which the telescope is suspended, re- 
volves ; and by a chain running over pulleys, and a weight and windlass, it is balanced, 
and raised or lowered. The lower end rests on a small carriage, that runs upon a circu- 
lar railroad around the tower, at the distance of 52 feet from its center. By these 
means it is directed to almost any point in the heavens. It is called the " Craig" tele- 
scope, in honor of Rev. Mr. Craig, under whose direction, and at whose expense, it was 
constructed. It is located at Wandsworth Common, near London. 



525. Describe the Craig telescope. Object-glass? — focal distance? Tube? 
(How mounted ? Why called " Craig" telescope ? Where located ?) 



DIFFERENT KINDS OF TELESCOPES. 



229 



526. Besides this monster refractor, there are several 
other very fine instruments in Europe ; as the Dorpat 
telescope, Sir James South's, the Northumberland re- 
fractor, the Oxford telescope, &c. Several colleges and 
seminaries in the United States have observatories con- 
nected with them, and telescopes of greater or less value. 
The largest is at Cambridge, near Boston. 

PUBLIC OBSERVATORIES AND TELESCOPES IN THE UNITED STATES: 



Observatories. 



Yale College 

Wesleyan University 

Williams College 

Hudson, Ohio 

Philadelphia , 

West Point 

Washington 

Cincinnati 

Cambridge 

Dartmouth College 

Georgetown " 

Erskine " ..... 

Shelby " 

Columbia (8. C.) College 
Columbia (Mo.) " 





THEIR TELESCOPES. 




When 
procured. 


Name of 
maker. 


Focal 
length. 


Aperture 
of object- 
glass. 


Cost. 






ft. in. 


inches. 




1S30 


Dollond. 


10 — 


5 


$1,000 


1S36 


Lerebonrs. 


7 — 


6 


1,000 


(1S36 
i 1S52 


Holcomb. 


10 — 


reflector 




A. Clark. 


9 — 


7 




1837 


Simms. 


5 6 


4 




1S40 


Merz. 


8 4 


n 


1,900 


1841 


Lerebours. 


8 — 


6 




1844 


Merz. 


15 3 


9-6 


6,000 


" 


u 


17 — 


12 


9,437 


1S46 


u 


22 6 


15 


19,842 


1S48 


u 


9 — 


6-4 




1S49 


Simms. 


7 6 


4-8 


1,600 


" 


Fitz. 


7 — 


5-6 


1,050 


1S50 


Merz. 


10 4 


7-5 


3,500 


1851 


Fitz. 


8 4 


H 


1,200 


1S52 


a 


5 — 


4 


225 



527. Quite a number of very respectable private ob- 
servatories are also in operation in different parts of the 
country. The following table includes most of them : 

PRIVATE .OBSERVATORIES. 



Private observatories. 



J. Jackson, near Philadelphia 

Mr. Longstreet, Philadelphia 

S. G. Gummere, Burlington, N. J.. 



When 
procured. 



1846 



1S47 
1S50 
1851 



E. Vanarsdale, Newark, N. J 

W. S. Van Duzee, Buffalo, N. Y | " 

W. S. Dickie, Elkton, Ky 

D. Mosman, Bangor, Me " 

J. Campbell, New York I 1S52 



THEIR TELESCOPE8. 



Maker. 



Fitz. 



Focal 
lerjgtk. 



ft. 

s 

7 
5 
7 
8 

11 
6 
5 

10 



in. 



Object- 
glass 

nches. 
6 3-10 
5 
4 
5 
6f 

a* 

4 
8 



Cost. 



$1,833 
900 
425 
750 
1,000 
2.220 
300 
225 
1,150 



J 

526. What other refractors in Europe besides the Craig ? Public observa- 
tories in this country ? Largest telescope ? Table. 

527. Private observatories— names ? Telescopes— by whom mostly made ? 



230 



ASTRONOMY. 



A COMET SEEKER. 



528. A Comet Seeker is a 
refracting telescope with a 
large aperture and short fo- 
cal distance. As comets 
cannot be found by their 
right ascension and declina- 
tion, but often have to be 
searched up, by sweeping 
around the heavens with a 
telescope, before they be- 
come visible to the naked 
eye, it is important to have telescopes that will cover 
considerable space — that is, of wide aperture and short 
focal distance. Such a telescope was made by Mr. Fitz 
for Miss Mitchel, of Newport, K. I. 




KEFLECTING TELESCOPES. 

529. The Reflecting Telescope is one in which the light 
is converged to a focus by means of a concave metallic 
reflector or speculum. Like the Refractors, they may 
be constructed with very little mounting; though, for 
convenience in use, it is necessary to place the reflector 
in a tube. 



SIMPLEST FORM OF A REFLECTING TELESCOPE. 




In this cut, the light A is seen passing from the object on the right, and falling upon 
the concave surface of the reflector at B, from which it is reflected back to a focus, and 
enters the eye of the observer at C. This telescope has no eye-piece. 

530. The focal distance of a concave reflector is equal 
to half the radius of the sphere formed by the concave 

528. What is a comet seeker f Why necessary ? 

529. Describe a reflecting telescope. Simplest form ? 

530. Focal distance ? (Diagram.) . 



REFLECTING TELESCOPES. 



231 



surface produced. Hence to grind a reflector for a focus 
of 20 feet, it will be necessary to have the curve that of 
a circle whose radius is 40 feet. 



FOCAL DISTANCE OF A CONCAVE REFLECTOR. 



B 



B ~~~~~-~-~^ 


e^~~~~— — ^ "*" 














§~~~~^— -^T: 


- — T~-~-^ 


^^ 












— = 5 


ssfeg 


gl^ 




A 


i\ • 


l^rzr^ 


^r^r 


s=^^ 




< «3* 




c . 


a— — -=• — ~~~~^^~^^-^~' 


|§ -~~~s^-"''~ 


1^-^' 



Here the curve of the speculum B is that of a circle, whose cen- 
ter is C ; while the focus of the speculum is at D, which is only- 
half the distance from B to C. 



*-£ 



531. Reflecting telescopes are of several kinds — viz., 
the Gregorian, the Newtonian, the Cassegranian, the 
Herschelia?i, &c. The Gregorian Reflector has an aper- 
ture in the center of the speculum, and a small concave 
mirror in the focus of the speculum, which reflects the 
light back through the aperture to the eye-piece. In 
this way the observer is enabled to face the object, and 
to direct the telescope toward it, as if it were a refractor. 



GREGORIAN REFLECTOR. 




1' 



Here the light A falls upon the speculum at B, and is reflected back to the small mir- 
ror C. by which it is thrown out, through the aperture in the speculum, to the eye of 
the observer at D. The object is supposed to be off on the right, in the direction toward 
which the instrument is pointed. It is called a Gregorian telescope, after Mr. James 
Gregory, who first suggested the construction of reflecting telescopes. 

532. The Newtonian Reflector is so called after Sir 
Isaac Newton, its inventor. Instead of a concave mir- 
ror in the focus of the speculum, he placed a plane mir- 



531. How many kinds of reflectors ? Describe the Gregorian. (Diagram, 
Why called Gregorian ?) 

532. Newtonian reflectors ? (Diagram and explanation.) 



232 ASTEONOMY. 



ror there, inclined so as to reflect the light to the side of 
the tube, when it was received by the observer. 



NEWTONIAN REFLECTOR. 




The light from the speculum is here shown falling upon the inclined mirror in the 
center, and reflected out to the eye of the observer. 

533. The Cassegranian Reflector is so called from M. 
Cassegrain, its inventor. It resembles the Gregorian, 
except that the speculum placed in the focus of the re- 
flector is convex, instead of concave. 

534. The Herschelian Reflector differs from all others, 
in having no small reflector whatever ; the light being 
reflected back to a focus at the top of the telescope, and 
near the edge of the tube, where the eye-piece is placed, 
and where the observer sits looking into the mirror with 
his back to the object. 

HERSCHELIAN TELESCOPE. 




Here the concave speculum is seen to be inclined a little to the lower side of the 
tube, so that the parallel rays A are reflected back to the observer at B, at the side of 
the instrument, where the eye-piece is placed. 

535. The first telescope constructed upon this plan was 
that by Sir William Herschel, in 1782. This was called 
his 20 feet reflector, and was the instrument with which 
he made many of his observations upon the double stars. 
In 1789, he completed his forty feet reflector, until 
recently the largest telescope ever constructed. 

533. Cassegranian ? Difference ? 

534. Herschelian? Where eye-piece ? How observer sit ? 

535. First Herschelian telescope ? What called ? Next ? 



REFLECTING TELESCOPES. 



233 




SIR WILLIAM HERSCHEL'S FORTY FEET REFLECTOR. 

536. The speculum of this instrument is 4 feet in 
diameter, 3J inches thick, and weighed, before being 
ground, 2,118 pounds. The tube is made of sheet iron 
riveted together, and painted within and without. 

The length of the tube is 39 feet 4 inches, and its weight 8,260 pounds. It is elevated 
or lowered by tackles, attached to strong frame-work ;"and the observer, who sits in a 
chair at the upper end of the tube, andlooks down into the reflector at the bottom, is 
raised and lowered with the instrument. Three persons are necessary to use this tele- 
scope — one to observe, another to work the tube, and a third to note down the observa- 
tions. A speaking tube runs from the observer to the house where the assistants are at 
work. By this telescope, the sixth and seventh satellites of Saturn were discovered ; 
and it was the chief instrument used by its distinguished owner, in making the obser- 
vations and discoveries which have immortalized his name, and which have so abun- 
dantly enriched and advanced the science of astronomy. 



536. Herschel's forty feet reflector ? Size of speculum ? Weight ? Tube ? 
Length and weight ? How mounted ? Observer where ? Usefulness ? 



234 



ASTRONOMY 




LORD ROSSE'S GREAT REFLECTING TELESCOPE. 

537. This is the largest reflecting telescope ever con- 
structed. The speculum, composed of copper and tin, 
weighed three tons as it came from the mould, and lost 
about ith of an inch in grinding. It is 5J inches thick, 
and 6 feet in diameter. It was cast on the 13th of April, 
1842, and was cooled gradually in an oven for 16 weeks, 
to prevent its cracking, by a sudden or unequal reduc- 
tion of the temperature. This speculum has a reflecting 
surface of 4,071 square inches. The tube is made of 
deal wood, one inch thick, and hooped with iron. Its 
diameter is seven feet, and its length 56. 

The entire weight of this telescope is twelve tons. 
It is mounted between two north and south walls, 24 feet 
apart, 72 feet long, and 48 feet high. The lower end 
rests upon a universal hinge. It can be lowered to the 
horizon, and raised to the zenith, and lowered northward 
till it takes in the Pole star. Its motion from east to 
west is limited to 15 degrees. This magnificent instru- 
ment is situated at Burr Castle, Ireland. It was con- 
structed by the Earl of Rosse, at an expense of $60,000. 

537. Lord Rosse's telescope ? Weight of speculum ? Diameter? Thick- 
ness ? Cooling ? Tube ? Entire weight ? How mounted ? What motion ? 
Where located? Cost? 



TRANSIT INSTRUMENT. 



235 




A TRANSIT INSTRUMENT. 



538. A Transit Instrument is a telescope used for ob- 
serving the transit of celestial objects across the meri- 
dian, for the purpose of determining differences of right 
ascension, or obtaining correct time. They are usually 
from six to ten feet long, and are mounted upon a hori- 
zontal axis, between two abutments of mason work ; so 
that the instrument, when horizontal, will point exactly 
to the south. It w T ill then take objects in the heavens, 
when they are exactly on the meridian. 

Let A D in the cut represent the telescope, and E and W the east and west abutments, 
between which it is 'placed. On the left is seen, attached to the mason work, a gradu- 
ated circle : and on the eastern end of the axis of the telescope is seen an arm, ??, ex- 
tending to the circle, as an index. Now, suppose the index n to be at o, in the upper 
part of the circle, when the telescope is horizontal ; then if the meridian altitude of the 
object to be taken is 10°, the index must be moved 1(P from o, as the degrees on the 
circle and the altitude of the object will correspond. 

538. What is a transit instrument ? Size ? How mounted ? (Describe 
parts as shown in the cut. How set the instrument for the meridian altitude 
of a star ?) 



Xu 



236 



ASTRONOMY. 



539. An Astronomical Clock is a clock adapted to keep 
exact sidereal time (136). 

Taking the vernal equinox in the heavens as the zero point, and reckoning 24 hours 
eastward to the same point again, the time — reckoning 15° to an hour — when an object 
crosses the meridian, will always represent the right ascension of the object. Hence 
right ascension is usually given in hours, minutes, and seconds ; or in time by the 
astronomical clock, set by the vernal equinox. 




THE MURAL CIRCLE. 



540. A Mural Circle is a large graduated circle, with 
a telescope crossing its center, used for the measurement 
of the altitudes and zenith distances of the heavenly 
bodies, at the instant of their crossing the meridian. 
They are usually fixed upon a horizontal axis, that turns 
in a socket firmly fixed in a north and south wall. The 
degrees, minutes, and seconds on the circle are read by 
means of microscopes, and indicate the altitude of the 
object. 

In the* cut, A is a reading microscope, and B C D E the wall to which the circle is at- 
tached. The telescope would denote an altitude of about 40°, which would leave 50° as 
the zenith distance. 

539. An astronomical clock ? How set ? How indicate right ascension of 
objects ? 

540. Describe a mural circle ? Its uses ? How mounted ? (How ascertain 
altitude and zenith distance by it ?) 



PARALLAX OF THE HEAVENLY BODIES. 237 



541. Parallax is the difference between the altitude of 
any celestial object seen from the earth's surface, and the 
altitude of the same object seen at the same time from 
the earth's center ; or it is the angle under which the 
semi-diameter of the earth w T ould appear, as seen from 
the object. 

The true place of a celestial body is that point of the 
heavens in which it w r ould be seen by an eye placed at 
the center of the earth. The apparent place is that point 
of the heavens where the body is seen from the surface 
of the earth. The parallax of a heavenly body is great- 
est when in the horizon, and is thence called the hori- 
zontal parallax. Parallax decreases as the body ascends 
toward the' zenith, at w r hich place 

. . , . ' -L PAEALLAX OF THE PLANETS. 

it is nothing. F 

The adjoining cut will afford a sufficient illustration. 
When the observer, standing upon the earth at A, 
views the object at B, it appears to be at C, when, at 
the same time, if viewed from the center of the earth, 
it would appear to be atD. The parallax is the angle 
BCD or A B E, which is the difference between the 
altitude of the object B, when seen from the earth's 
surface, and when seen from her center. It is also 
the angle under which the semi-diameter of the earth, l///^/ \a^\ 

A E, is seen from the object B. lAy?/^-----^^^^ 

As the object advances from the horizon to the ze- ^w^~^^^^ \ 

nith, the parallax is seen gradually to diminish, till at /^ — : — — -__-B ij) 

F it has no parallax, or its apparent and true place are f \mffffi " " — ^C 

the same. ^Mgy 

This diagram will also show why objects nearest *^^ 
the. earth have the greatest parallax, and those most 

distant the least ; why the moon, the nearest of all the heavenly bodies, has the greatest 
parallax; while the fixed stars, from their immense distance, have no appreciable 
horizontal parallax — the semi-diameter of the earth, at such a distance, being no more 
than a point. 

542. As the effect of parallax on a heavenly body is 
to depress it helow its true place, it must necessarily affect 
its right ascension and declination, its latitude and longi- 
tude. On this account, the parallax of the sun and 
moon must be added to their apparent altitude, in order 
to obtain their true altitude. 

The true altitude of the sun and moon, except when in the zenith, is always affected, 
more or less, both by parallax and refraction, but always in a contrary manner. Hence 
the mariner, in finding the latitude at sea, always adds the parallax, and subtracts the 
refraction, to and from the sun's observed altitude, in order to obtain the true altitude, 
and thence the latitude. 

541. Parallax? True place of a celestial body ? Apparent? When par- 
allax greatest ? Least? Called what, and why ? (Diagram? What objects 
greatest parallax ?) 

542. Effect of parallax ? How obtain true altitude ? (How differ from re- 
fraction ? How then obtain true altitude ?) 



238 ASTRONOMY. 



543. The principles of parallax are of great import- 
ance to astronomy, as they enable us to determine the 
distances of the heavenly bodies from the earth, the mag- 
nitudes of the planets, and the dimensions of their or- 
bits. 

The sun's horizontal parallax being accurately known, 
the earth's distance from the sun becomes known ; and 
the earth's distance from the sun being known, that of 
all the planets may be known also, because we know the 
exact periods of their sidereal revolutions, and, according 
to the third law of Kepler, the squares of the times of 
their revolutions are proportional to the cubes of their 
mean distances. Hence, the first great desideratum in 
astronomy, where measure and magnitude are concerned, 
is the determination of the true parallax. 

At a council of astronomers assembled in London some years since, from the most 
learned nations in Europe, the sun's mean horizontal parallax was settled, as the result 
of their united observations, at 0° 0' 8".5776. Now the value of radius, expressed like- 
wise in seconds, is 206264".S; and this divided by 8".5776, gives 24047 for the distance 
of the sun from the earth, in semi-diameters of the latter. If we take the equatorial 
semi-diameter of the earth as sanctioned by the same tribunal, at (7924-r-2=)3962 miles, 
we shall have 24047X3962=95,273,869 miles for the sun's true distance. 

544:. The change in the apparent position of the fixed 
stars, caused by the change of the earth's place in her 
revolution around the sun, is called their annual paral- 
lax. So immense is their distance, that the semi-annual 
variation of 190,000,000 of miles in the earth's distance, 
from all those stars that lie in the plane of her orbit, 
makes no perceptible difference in their apparent magni- 
tude or brightness. 

The following cut will illustrate our meaning : 



B 
, ' IZ -£ _ A 



B 

Let A represent a fixed star in the plane of the earth's orbit, B. At C, the earth is 
190 000 000 of miles nearer the star than it will be at D six months afterward; and yet 
this semi-annual variation of 190,000,000 miles in the distance of the star is so small a 
fraction of the whole distance to it, as neither to increase or diminish its apparent 
brightness. 



543. Use of parallax ? How employed ? (Note?) # 

544. What meant by earth's annual parallax ? Effect of variation of earth s 
distance on the fixed stars ? (Diagram.) 



MISCELLANIA. 239 



545. It is only those stars that are situated near the 
axis of the earth's orbit whose parallax can be measured 
at all, on account of its almost imper- parallax of the staes 
ceptible quantity. So distant are 

they, that the variation of 190,000,000 \ / 

miles in the earth's place causes an 
apparent change of less than V in B ..*, 

the nearest and most favorably situ- / \ 

ated fixed star. / - \ 

Let A represent the earth on the 1st of January, and B / \ 

a star observed at that time. Of course, its apparent place 
in the more distant heavens will be at C. But in six / 

months the earth will be at D, and the star B will appear 

to be at E. The angle ABD or CBE will constitute / 

the parallatic angle. ^ln the cut, this angle amounts to />- <\ 

about 4S°, whereas the real parallax of the stars is less D$ -& 9 A 

than ^!-th of one degree, or --^L-th part this amount. *\ ___.---''' 

Lines approaching each other thus slowly would appear 
paral : el ; and the earth's orbit, if filled with a globe of fire, 
and viewed from the fixed stars, would appear but a point of light 1' in diameter! 

MISCELLANIA. 

546. The Atmosphere is an elastic gas, which sur- 
rounds the earth on every side. It is supposed to be 
from 40 to 60 miles in hight, growing more rare as we 
ascend, and is kept around the earth by attraction. 

547. Wind is air put in motion by heat, causing bodies 
of air to rise from the earth's surface, and other air to 
rush in to supply its place. The velocity of the wind 
ranges from 5 to 100 miles an hour. 

548. Clouds are collections of vapor suspended in the 
air. They range from two miles to half a mile in hight, 
according to their density and weight. They serve to 
screen us from the oppressive heat of the sun, and to 
convey water from the rivers and oceans, and pour it 
down in showers upon the earth. 

549. Rain is water condensed, or collected into drops 
by attraction, and falling from the clouds. Hail is drops 

545. What stars have perceptible parallax? Amount? (Diagram, and 
explain.) 

546. What is the atmosphere ? Extent ? How kept around the earth? 

547. Wind ? How put in motion ? Velocity ? 

548. Clouds? Uses? 

549. Eain? Hail? Snow? 



240 ASTRONOMY. 



of rain frozen on its way to the earth ; and Snow is par- 
ticles of clouds frozen before being condensed into drops. 

550. Lightning is electricity passing from one cloud 
to another, or between the clouds and the earth ; and 
Thunder is the sudden shock given to the atmosphere 
by the passage of the electricity through it. 

551. The Aurora Borealis, or Northern Light, is a 
reddish unsteady light sometimes seen in the north. It 
is supposed to be caused by electricity passing through 
the upper regions of the atmosphere, about the North 
Pole. 

552. " Shooting Stars" are meteors that shoot down- 
ward toward the earth, like stars falling from their 
spheres. They are usually seen one at a time, and only 
in the night, but sometimes fall in showers, and no doubt 
fall in the day time, though invisible. 

From 2 o'clock in the morning, November 13, 1S83, till daylight, the whole heavens 
were filled with these fiery particles and streaks of light darting downward from the 
sky. These meteors, no doubt, come from regions beyond the limits of the atmosphere, 
and are ignited by their rapid passage through it. Their origin and nature are as yet 
matters of inquiry and speculation. . 

553. Aerolites, or Meteoric Stones, are masses of stone 
or iron that have fallen from the sky at various periods, 
and on almost every part of the globe. They are often 
found after the explosion of large meteors, sometimes 
while they are yet hot. 

A large meteor exploded over Cabarras county, North Carolina, a few years since, 
several pieces of which were picked up the next day. One piece, weighing*19 lbs., had 
struck a large pine tree lying on the ground, and had gone through it, and into the 
earth, to the depth of three' feet. In some cases, large masses of iron have fallen. In 
December, 1795, a stone weighing 51 lbs. fell in Yorkshire, England. The writer has a 
piece of an aerolite that weighed 90 lbs., that fell in New Jersey. A large mass of 
meteoric iron may be seen in the museum of Yale College. 

550. Lightning and thunder? 

551. Aurora Eorealis ? 

552. "Shooting stars?" How seen? (What shower mentioned? Dis- 
tance from which they come ?) 

553. Aerolites ? (What instances of their fall cited ?) 




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